EP1685281A2 - Membrane -mediated electropolishing - Google Patents

Membrane -mediated electropolishing

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Publication number
EP1685281A2
EP1685281A2 EP04800491A EP04800491A EP1685281A2 EP 1685281 A2 EP1685281 A2 EP 1685281A2 EP 04800491 A EP04800491 A EP 04800491A EP 04800491 A EP04800491 A EP 04800491A EP 1685281 A2 EP1685281 A2 EP 1685281A2
Authority
EP
European Patent Office
Prior art keywords
membrane
conductivity
metal
low
work piece
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP04800491A
Other languages
German (de)
English (en)
French (fr)
Inventor
Stephen Mazur
Charles E. Jackson, Jr.
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
EIDP Inc
Original Assignee
EI Du Pont de Nemours and Co
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by EI Du Pont de Nemours and Co filed Critical EI Du Pont de Nemours and Co
Publication of EP1685281A2 publication Critical patent/EP1685281A2/en
Withdrawn legal-status Critical Current

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Classifications

    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25F—PROCESSES FOR THE ELECTROLYTIC REMOVAL OF MATERIALS FROM OBJECTS; APPARATUS THEREFOR
    • C25F3/00—Electrolytic etching or polishing
    • C25F3/16—Polishing
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23H—WORKING OF METAL BY THE ACTION OF A HIGH CONCENTRATION OF ELECTRIC CURRENT ON A WORKPIECE USING AN ELECTRODE WHICH TAKES THE PLACE OF A TOOL; SUCH WORKING COMBINED WITH OTHER FORMS OF WORKING OF METAL
    • B23H5/00—Combined machining
    • B23H5/06—Electrochemical machining combined with mechanical working, e.g. grinding or honing
    • B23H5/08—Electrolytic grinding
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B24—GRINDING; POLISHING
    • B24B—MACHINES, DEVICES, OR PROCESSES FOR GRINDING OR POLISHING; DRESSING OR CONDITIONING OF ABRADING SURFACES; FEEDING OF GRINDING, POLISHING, OR LAPPING AGENTS
    • B24B37/00—Lapping machines or devices; Accessories
    • B24B37/04—Lapping machines or devices; Accessories designed for working plane surfaces
    • B24B37/042—Lapping machines or devices; Accessories designed for working plane surfaces operating processes therefor
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B24—GRINDING; POLISHING
    • B24B—MACHINES, DEVICES, OR PROCESSES FOR GRINDING OR POLISHING; DRESSING OR CONDITIONING OF ABRADING SURFACES; FEEDING OF GRINDING, POLISHING, OR LAPPING AGENTS
    • B24B37/00—Lapping machines or devices; Accessories
    • B24B37/04—Lapping machines or devices; Accessories designed for working plane surfaces
    • B24B37/046—Lapping machines or devices; Accessories designed for working plane surfaces using electric current
    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25F—PROCESSES FOR THE ELECTROLYTIC REMOVAL OF MATERIALS FROM OBJECTS; APPARATUS THEREFOR
    • C25F3/00—Electrolytic etching or polishing
    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25F—PROCESSES FOR THE ELECTROLYTIC REMOVAL OF MATERIALS FROM OBJECTS; APPARATUS THEREFOR
    • C25F3/00—Electrolytic etching or polishing
    • C25F3/02—Etching
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/04—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer
    • H01L21/18—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer the devices having semiconductor bodies comprising elements of Group IV of the Periodic Table or AIIIBV compounds with or without impurities, e.g. doping materials
    • H01L21/30—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26
    • H01L21/31—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26 to form insulating layers thereon, e.g. for masking or by using photolithographic techniques; After treatment of these layers; Selection of materials for these layers
    • H01L21/3205—Deposition of non-insulating-, e.g. conductive- or resistive-, layers on insulating layers; After-treatment of these layers
    • H01L21/321—After treatment
    • H01L21/32115—Planarisation
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/04—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer
    • H01L21/18—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer the devices having semiconductor bodies comprising elements of Group IV of the Periodic Table or AIIIBV compounds with or without impurities, e.g. doping materials
    • H01L21/30—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26
    • H01L21/31—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26 to form insulating layers thereon, e.g. for masking or by using photolithographic techniques; After treatment of these layers; Selection of materials for these layers
    • H01L21/3205—Deposition of non-insulating-, e.g. conductive- or resistive-, layers on insulating layers; After-treatment of these layers
    • H01L21/321—After treatment
    • H01L21/32115—Planarisation
    • H01L21/3212—Planarisation by chemical mechanical polishing [CMP]
    • H01L21/32125—Planarisation by chemical mechanical polishing [CMP] by simultaneously passing an electrical current, i.e. electrochemical mechanical polishing, e.g. ECMP

Definitions

  • This invention relates to a process for electropolishing metal surfaces, and more specifically to a membrane mediated electropolishing process, wherein a conductive electrolyte contacts a cathode and a first side of a charge-specific ion-conducting membrane, the membrane movably contacts the work piece anode having a low- conductivity fluid disposed thereon, and on applying sufficient voltage a current flows between the cathode and the work piece, metal is removed from the surface by means of electrochemical anodic oxidation, and most of this metal passes through the membrane and into the electrolyte.
  • the invention relates to use of membrane mediated electropolishing to polish copper damascene wafers at rate variable from less than 50 niA/cm to more than 3000 mA/cm of membrane contact area, wherein metal is removed from the surface by means of electrochemical anodic oxidation and through a membrane contacting the metal surface that is permeable to the removed metal ions, and wherein the metal ions pass through the membrane and into the electrolyte, and therefore do not contaminate the wafer surface.
  • Conventional electropolishing (EP) is a common metal finishing process in which the metal object to be polished (the "work piece") is connected to the positive terminal of a DC electrical power supply as the anode.
  • the surface of the work piece or some portion of that surface is brought into contact with an electrolyte solution which, in turn, also contacts a second electrode (the cathode) which is connected to the negative terminal of the power supply.
  • a suitable voltage difference is applied between the two electrodes, or a suitable current density is established at the anode, then the work piece undergoes anodic oxidation forming solvated metal ions that dissolve into the electrolyte.
  • current density is defined as the electrical current per unit surface area of anode in contact with the electrolyte and has units such as mA/cm 2 ).
  • EP is capable of producing a mirror-like reflective finish on many different kinds of metals and is especially useful for polishing metal parts with curved surfaces and complicated shapes.
  • D.E. Ward pp. 100-120 in Electroplating Engineering Handbook. 4 th Edition, L.J. Durney, Ed., Nan ⁇ ostrand Co., ⁇ Y, 1984
  • the rate of the EP process is limited by mass transport of molecules and/or ions in the electrolyte close to the anode surface and is generally optimized by convection of the electrolyte.
  • Recent interest has focused on the use of EP to selectively remove excess copper in the fabrication of integrated circuits via the copper damascene process.
  • a silicon wafer is uniformly covered with a dielectric layer, for example ⁇ 0.5 microns of SiO .
  • a pattern corresponding to the conductive circuit elements is then etched through the dielectric layer by photo-lithographic methods, and the entire surface is coated with a thin "barrier" layer, for example ⁇ 10nm tantalum or tantalum nitride.
  • a layer of copper is then grown over the entire surface of the wafer by means of electroplating.
  • This copper "blanket” must be sufficiently thick to fill the etched circuit features (-0.5 microns), but is generally not thicker than about 1 micron in total thickness.
  • the external surface of the copper blanket generally retains topographic features that conform to the larger features in the underlying etched pattern.
  • the next step in the process requires removing all excess copper from the surface of the barrier- coated dielectric while leaving the etched circuit elements filled with copper.
  • the final surface must be left planar to within very narrow tolerances in order to permit subsequent fabrication of addition layers of circuitry.
  • the cross-section of a typical copper damascene wafer before and after polishing and planarization is shown schematically in Figure 1.
  • Planarization and removal of excess copper from Cu damascene wafers is currently achieved by means of chemical-mechanical polishing (CMP), involving mechanical abrasion and chemical reactions with oxidizers and other chemicals.
  • CMP is a costly process, generates hazardous waste products, and is incompatible with the mechanically fragile materials currently under development for improved dielectric layers.
  • EP has been considered as an alternative to overcome the limitations CMP.
  • planarization is meant the ability to preferentially remove topographic high points (plateaus or ridges) of both large and small lateral dimensions so that the polished surface progressively approaches an ideal plane. Planarizing conventionally plated Cu damascene wafers requires especially high efficiency. "Planarization efficiency" maybe defined quantitatively by reference to Figure 2.
  • planarization efficiency of a polishing process is defined by the derivative da/d ⁇ , namely the differential change in amplitude of a topographic feature with the change in average thickness.
  • Electrolyte convection increases the rate of EP by enhancing mass transport of solubilizing molecules such as water to the surface of the work piece (see for example S. H. Glarum , J.H. Marshall, J Electrochem. Soc, 132, 2872 (1985)). More efficient mixing of the electrolyte produces a thinner convective boundary layer resulting in faster mass transport.
  • C. Wagner J Electrochem. Soc, 101, 225 (1954)
  • One aspect of the invention relates to an apparatus for and a method of membrane-mediated electropolishing of a substrate, which is advantageously a metal layer on a copper damascene wafer used in the manufacture of integrated circuits, but also can be used on a variety of other metals and on other applications, such as planarizing and polishing other metals disposed on integrated circuit wafers, and for planarizing and polishing antenna, a micromachine, a read/write head, a memory or rigid disk, or other substrates where the tolerances are demanding.
  • the process of membrane mediated electropolishing of an area A p of a substrate comprises the steps of: a) providing a substrate having a surface with a metal disposed thereon, wherein said metal is electrically connected to the positive terminal of a substantially DC electric power supply; b) providing a low-conductivity fluid layer to the surface, wherein the low- conductivity fluid advantageously has a low conductivity below about 1000 ⁇ S/cm, preferably below about 500 ⁇ S/cm, more preferably below about 200 ⁇ S/cm, for example between about 0.5 ⁇ S/cm and about 150 ⁇ S/cm, typically below about 100 ⁇ S/cm, even more preferably below 50 ⁇ S/cm, for example between about 0.1 and about 10 ⁇ S/cm, alternately between about 1 ⁇ S/cm and about 10 ⁇ S/cm; c) providing a cathode half-cell including: 1) a cathode connected to the negative terminal of a substantially DC electrical power supply
  • Membrane-mediated electropolishing is in part distinguished by the following characteristics.
  • a membrane disposed between a cathode and the work piece comprises first and second sides, wherein on the first side a conductive electrolyte composition contacts both the membrane and the cathode, and on the second side a low- conductivity fluid contacts the work piece and the membrane. Ionic current flow between the cathode and the work piece passes through the half-cell fluid, the membrane, and the low-conductivity fluid.
  • the membrane is substantially impermeable to the electrolyte solutes (and acids) in the electrolyte composition, but a substantial portion of the metallic material electropolished from the work piece migrates through the membrane toward the cathode and into the conductive electrolyte composition.
  • Another aspect of the invention relates to a method of membrane-mediated electropoUshing of a work piece having a surface comprising a metal, the method comprising: A) providing the work piece surface substantially covered with a low- conductivity fluid, wherein the work piece is in electrical contact with a first positive outlet of a substantially DC electric power source having positive and negative power outlets; B) substantially and movably contacting the work piece surface covered with the low-conductivity fluid with a first side of an ion-conducting membrane having first and second sides, wherein the second side of the ion-conducting membrane contacts a conductive electrolyte composition having a conductivity greater than 10 mS/cm, and the ion-conducting membrane prevents the conductive electrolyte composition from contacting the work piece; and C) electrically contacting the conductive electrolyte composition with a cathode, wherein the cathode is electrically connected to the negative outlet of the substantially DC electric power source, thereby causing current to flow between the work
  • substantially contacting it is meant that the membrane is contacting the work piece, although a thin layer, e.g., a boundary layer, of low-conductivity fluid may exist between the membrane and the work piece, or an "effective" boundary layer may exist withing the pores of the membrane.
  • a thin layer e.g., a boundary layer
  • the ion-conducting membrane is a charge-specific ion-conducting membrane;
  • the conductive electrolyte composition has a conductivity equal to or greater than about 5mS, preferably equal to or greater than about 30 mS/cm, more preferably equal to or greater than about 100 mS/cm;
  • the low-conductivity fluid has a conductivity below about 500 ⁇ S/cm, preferably below 100 ⁇ S/cm, for example ⁇ 10 ⁇ S or between about 0.1 and about 10 ⁇ S/cm.
  • substantially all of the metal ions electropolished from the work piece pass through the membrane and into the conductive electrolyte composition.
  • ion-exchange material i.e., material having ability to substantially capture and retain the metal-containing ions originating from the metal or from other sources, disposed into the electric current pathway between the cathode and the work piece surface.
  • An "ion-exchange" material is a functional term implying the membrane captures an ion, at the same time releasing another type of ion.
  • the membranes of the current invention 1) in some embodiments the membrane does not have a substantial number of functional moieties to capture metal ions as required by an ion exchange material, 2) in most embodiments the membrane is nearly saturated with metal ions, and thus has no "ion-exchange" capacity as is known in the art, and finally, 3) in all embodiments the "ion-exchange" capability is too low to be useful in functioning as such during electropolishing. Rather, the membrane in the current invention is an ion-conductive material, and is typically a charge selective ion- conductive material. Removing 1 micron of Cu from a single 20 cm Cu damascene wafer generates 4.4 mMoles of Cu +2 .
  • a preferred membrane for use in the MMEP process are any of the commercially available National ® membranes, and a 5 cm 2 layer of the thickest National ® membrane (Nl 17, with Equivalent weight 1100), has a total exchange capacity of only 81 microMoles of Cu +2 . That is, the membrane would be saturated with Cu after polishing only a few percent of the required depth. This is, incidentally, a major difficulty with prior art polishing processes that utilize ion exchange material in the polishing.
  • the process of electropolishing a metal ion, e.g., copper, and tranporting the metal ion through the membrane is believed to remove water or other solvent from the interface between the work piece and the membrane, and, under certain polishing conditions, the contact area can lose its boundary layer of water and stick to the work piece.
  • the electropolishing method advantageously further comprises introducing additional low-conductivity fluid to the interface between the membrane and the work piece surface during polishing.
  • the solvated electropolished ions have a positive charge
  • the ion-conducting membrane is a cation-conducting membrane comprising a polymeric ionomer functionalized with strong acid moieties having pKa(s) less than 5, preferably less than 3, for example a perfluorosulfonic acid/PTFE copolymer or a perfluorocarboxylic acid/PTFE copolymer.
  • the ion-conducting membrane is an anion-conducting membrane, for example a polymer functionalized with protonated amines and/or quaeternary ammom ' um ions.
  • the membrane further comprises a partial covering or structure disposed between the charge-selective ion-conducting membrane and the work piece, wherein said partial cover is substantially permeable to those ions to which the membrane is permeable and also to low-conductivity fluid, and wherein said partial covering has openings sufficiently large to allow the membrane to extend therethrough to "sensibly" contact the work piece surface, i.e., to contact the work piece surface albeit with perhaps a very thin boundary layer of water between the contacting surface of the membrane and the work piece surface.
  • This partial covering can vary widely in thickness and have a plurality of functions, as discussed infra.
  • the conductive electrolyte composition has a conductivity greater than about 100 mS/cm and comprises water, an acid, and a metal salt.
  • the conductive electrolyte composition comprises water and acetonitrile.
  • the low-conductivity fluid advantageously has a conductivity much lower than that of the conductive electrolyte composition, generally at least 100 times lower than, and preferably at least 1000 times lower than said conductive electrolyte composition (e.g., the low-conductivity fluid has a conductivity below about 200 ⁇ S/cm or below about 100 ⁇ S/cm).
  • the method can efficiently planarize most structures on a metal-containing integrated circuit surface if the low-conductivity fluid has a conductivity between about 0.5 ⁇ S/cm and about 150 ⁇ S/cm.
  • the low-conductivity fluid advantageously has a conductivity between about l ⁇ S/cm and about lO ⁇ S/cm.
  • the low-conductivity fluid may consists essentially of water, acetonitrile, propionitrile, butyronitrile, ethylene carbonate, propylene carbonate, glycol, ethylene glycol, propylene glycol, diethlyene glycol, or mixtures thereof.
  • the low-conductivity fluid may consist essentially of water and at least one of acetonitrile, propionitrile, butyronitrile, an alcohol having from 1 to 8 carbon atoms, glycerol, ethylene glycol, propylene glycol, diethlyene glycol, proplylene glycol monomethyl ether, propylene carbonate, ethylene carbonate, an alkanolamine, ethylenediamine, and dimethylformamide.
  • the solvating capacity of the low- conductivity fluid changes with pH, and the pH varies with polishing, so advantageously the low-conductivity fluid comprises a buffer in a quantity sufficient to maintain a pH during electropolishing of within about 0.2 pH units of an initial pH.
  • a cathode half-cell comprises: a fully or partially enclosed volume having a charge-specific ion-conducting membrane forming a surface of the volume, and wherein a cathode is disposed within the volume and a conductive electrolyte composition partially or essentially fills the volume and is in electrical contact with both the membrane and the cathode; a connector electrically connecting the cathode to the negative outlet of a substantially DC electric power source; and an inlet and an outlet flowingly connected to a pump adapted to circulate the conductive electrolyte composition between the enclosed volume.
  • the method can be performed as described above, and advantageously the pump can circulate conductive electrolyte composition between the half cell volume and the pump.
  • the circulating flow of conductive electrolyte composition into and out from the half cell is controlled, such that the pressure within the half cell is controlled at a preselected pressure that is typically between about 0.1 psig and about 40 psig, more typically between about 0.1 psig and about 20 psig.
  • the pump may be further flowingly connected to a second apparatus adapted to change the temperature of the electrolyte composition or to change the composition of the electrolyte composition, e.g., filter, degas, add components, etc.
  • the electrical power source provides a pulsed DC current alternating between a high current value and a low current value that is less than 30%, preferably less than 50%, more preferably less than 70%, of the high current value.
  • the period of alternating can, for example, be such that the low current value is maintained for a period of time between 0.00001 seconds (10 ⁇ sec) and 1 second and the high current value is maintained for a period of time between about 0.00001 seconds (10 ⁇ sec) and 5 seconds.
  • the pulsing current appears to increase planarization efficiency under some circumstances.
  • Various electrolyte compositions are useful in this method.
  • the electrolyte solvent may advantageously comprises one or more nitrogen-containing solvents that can stabilize Cu(I) ions, for example soluble nitriles such as acetonitrile, propionitrile, and/or butyronitrile, alkanolamines such as monoethanolarnine, diethanolamine, triethanolamine, and/or isopropanolamine, ammonia, ethylene diamine, or mixture thereof in an amount sufficient to give a electropolishing efficiency of one copper atom for every 1 to 1.99 electrons of current flow.
  • soluble nitriles such as acetonitrile, propionitrile, and/or butyronitrile
  • alkanolamines such as monoethanolarnine, diethanolamine, triethanolamine, and/or isopropanolamine
  • ammonia ethylene diamine, or mixture thereof in an amount sufficient to give a electropolishing efficiency of one copper atom for every 1 to 1.99 electrons of current flow.
  • the conductive electrolyte composition comprises water, one or more acids acid, and from about 0.001 M to about 1 M of a metal salt.
  • the metal from the metal salt is reducible at the cathode.
  • One side reaction intrinsic to electropolishing is the electrolysis of water, which can produce molecular hydrogen, molecular oxygen, or both.
  • an additional invention is a method of electropolishing or electro-mechanically polishing Cu damascene wafers wherein the electrolyte solution contacting the cathode contains a sufficient concentration of reducible-metal salts to substantially suppress the formation of molecular hydrogen at the cathode, but where the electrolyte or fluid contacting the Cu damascene wafers has substantially less reducible-metal salts.
  • Preferred reducible- metal ions are Cu(I), Cu(II), Ni(II), Ag(I), Fe(II), Cr(III), and/or complexes of these ions.
  • the most preferred reducible-metal ions are Cu(I) and Cu(II).
  • a concentration of between 0.001 M to about 1 M of a reducible-metal salt is sufficient to suppress the formation of molecular hydrogen at the cathode, and in preferred embodiments the electrolyte contacting the cathode comprises about 0.01 M to 0.5 M, for example about 0.05 M to about 0.3 M, alternately from about 0.1 M to about 0.3 M, of a reducible-metal salt.
  • the reducible metal from the metal salt may be the same as the metal electropolished from the work piece surface, or may be different than the metal electropolished from the work piece surface.
  • the electrolyte solution or gel comprises a reducible-metal salt, wherein the amount of reducible-metal salt is sufficient to substantially reduce the formation of hydrogen gas.
  • the conductive electrolyte solution or gel comprises from about 0.001 M to about 1 M, preferably about 0.01 M to 0.5 M, for example about 0.05 M to about 0.3 M, or alternately from about 0.1 M to about 0.3 M, of reducible-metal salt.
  • the low-conductivity fluid comprises or consists essentially of at least one solvent that can solvate a metal ion, e.g., one or more of water, alkanolamines, soluble nitriles, soluble diamines, amides, ethylene carbonate,r propylene carbonate, or any mixture thereof.
  • the low-conductivity fluid may comprises additives in low quantities, for example a chelator.
  • the low-conductivity fluid may comprise a polar organic solvent, which is particularly advantageous if the work piece comprises water-sensitive materials, such as low-K materials.
  • the low-conductivity fluid, the conductive electrolyte composition, or both may advantageously comprise a source of strong base, cyanide ion, and/or chloride ions.
  • the work piece surface may comprise copper, particularly useful for integrated circuit manufacture.
  • membrane mediated electropolishing is that the electropolished ions can be made to substantially quantitatively pass through the membrane and be trapped within a small reservoir of conductive electrolyte. Membrane mediated electropolishing is therefore beneficial if the work piece comprise environmentally hazardous metals, for example cobalt and/or chromium.
  • the work piece may comprise valuable metals such as Ag, Au, and also valuable metals such as Ir, Rh, and Ru, which can be collected and recycled.
  • metals and/or valuable metals such as vanadium, manganese, zirconium, niobium, molybdenum, hafnium rhenium, osmium, iridium, ruthenium, rhodium, and/or silver,can be beneficially polished by membrane mediated electropolishing.
  • the metal on the work piece surface may comprise other metals commonly found on semiconductor substrates, including aluminum, nickel, tantalum, titanium, or tungsten, or nitrides thereof.
  • the substrate surface also often comprises one or more dielectric materials. The method is useful for substrates having all commonly used dielectrics.
  • One advantage of this invention over conventional chemical mechanical polishing is the method of this invention does not appear to cause any attrition of dielectric layers.
  • the membrane-mediated electropolishing process comprises: A) providing a cathode half-cell comprising: 1. a fully or partially enclosed volume, cavity or vessel; 2. an electrolyte solution or gel which partially or essentially fills the enclosed volume, cavity, or vessel; 3. a cathode in contact with the electrolyte solution or gel; 4. a connector electrically comiecting the cathode to the negative terminal of a substantially DC power source; and 5.
  • a charge-selective ion-conducting membrane which seals one surface of the enclosed volume, cavity, or vessel in such a way that the internal surface of said membrane contacts the electrolyte solution or gel and that the external surface is adapted to movably contact the metal work piece; B) substantially covering a surface of a metal work piece with a low-conductivity solvent or solution; C) providing a source of DC electrical power whose positive terminal is connected to a metal work piece; and D) movaably contacting the metal work piece with at least a portion of the external surface of the membrane, while imposing a voltage between the cation and the work piece, thereby electropolishing metal from the work piece.
  • Another aspect of the invention relates to the apparatus adapted to perform the invention.
  • this invention also provides an apparatus for use in membrane-mediated electropolishing of a metal work piece, comprising: a. a substantially DC electric power source; b. a connector for electrically connecting the work piece to the positive terminal of the substantially DC electric power source; c. a conductive electrolyte solution or gel; d. a low conductivity solvent or solution which is in contact with a charge- selective ion-conducting membrane and with the work piece; e. a charge-selective ion-conducting membrane which has an internal surface which contacts the electrolyte solution or gel and an external surface which is accessible to contact the metal work piece and the low-conductivity solvent or solution disposed thereon; f.
  • This invention also provides a cathode half-cell for use in membrane-mediated electropolishing, comprising: a. a fully or partially enclosed volume, cavity, or vessel; b. an electrolyte solution or gel, which partially or substantially fills the enclosed volume, cavity, or vessel; c. a cathode which is in contact with the electrolyte solution or gel; d.
  • Membrane mediated electropolishing is a process for polishing and planarizing a metal work piece via anodic oxidation mediated by a charge-differentiating ion-conducting membrane.
  • Another aspect of the present invention relates to improvements in the MMEP process and specifically to improvements which provide for high rates of removal of metal from the work piece while minimizing precipitation of metal oxides, hydroxides, and/or carbonates, and minimizing contact pressure between the membrane and the work piece.
  • n could equal between about 2 and 3, often very close to 2.
  • n is advantageous in that electrolytic oxidation of water, and the accompanying evolution of gas is minimized, but the polishing rate was limited by the low voltages and subsequently lower current densities. Further, as the voltage and the polishing rate increased, a particle-like precipitate formed on the polished surface.
  • the particles can include copper oxide, copper hydroxide, copper carbonate, or mixture thereof.
  • the particles can include copper oxide, copper hydroxide, copper carbonate, or mixture thereof.
  • a polarization curve we polish substrates at varying voltages. When a substrate was polished at increasing voltages, e.g., as described in the examples, a dark coating of CuO would be clearly visible on the wafer at 7 volts.
  • These oxide coatings or particles could be abraded from the surface, for example by contacting the surface with a window-frame structure that advantageously moves across the surface of the substrate and applies an abrasive action sufficient to remove the particles.
  • the window-frame can advantageously move with the membrane, and may have abrasive attached thereto, in much the manner as an abrasive pad used in conventional CMP has abrasive attached thereto.
  • This type of polishing e.g., forming an oxide and abrading off the oxide, is similar to conventional CMP.
  • the window-frame structure can also advantageously be useful for supplying low-conductivity fluid to the substrate surface, or for dividing a large membrane into a plurality of smaller areas which contact the substrate and which can even be part of the circuit between the metal surface being polished and the positive terminal of the electrical power supply. Additionally, at lower voltages, e.g., voltages ⁇ 8N, the current density I decreased systematically with increasing velocity v and with decreasing contact pressure.
  • the tool had to be designed to provide a large area of contact of the membrane surface (A c ) relative to the area to be polished (A p ).
  • Such large contact area and higher hydrostatic pressures (P h ) increased the resultant tool pressure, herein defined as the downward force divided by the are to be polished (F z /A p ).
  • n is advantageously at least 3 electrons per copper atom, preferably at least 3.5, and more preferably 3.9 or greater.
  • the required number of protons necessary to prevent precipitation will to some extent depend on the polishing rate, which is directly proportional to the concentration of copper ions in the low-conductivity fluid.
  • a higher value of n is required for higher copper dissolution rates.
  • an n value in excess of 4, for example between about 4.5 and 8, will oxidize water molecules and thereby provide protons in an amount sufficient to prevent precipitation of copper oxides, copper hydroxides, and copper carbonates under substantially any commercially desirable polishing rate.
  • the solubility of copper ions in aqueous solutions depends upon the pH.
  • Fully protonated, Nation ® membranes are sufficiently acidic (e.g., pH ⁇ 2) to maintain a high concentration of copper ions in solution.
  • the pH may increase to values greater than 4 where the solubility of copper ions is extremely low. This may occur, for example, when copper ions are produced at the anode faster than protons can diffuse into the membrane from the electrolyte solution.
  • the membrane may be kept partially protonated so that the pH remains less than 4.
  • n is equal to or greater than 4, the number of protons produced by electrolysis of water (two or more per copper atom) are sufficient to maintain a state of protonation of the membrane such that precipitation of copper particles is not an issue.
  • the second benefit of polishing at high voltages is that I increases. Under conditions where ohmic resistance is larger, one would expect the slope dl/dN of the polarization curve of current density I versus voltage N would at best stay constant, e.g., so that doubling the voltage would at best double the current density. Also, since the rate of water electrolysis also increases with increasing N, and since the fraction of electrons utilized for oxidation of copper must decrease accordingly, it was not obvious that the rate of copper removal (RR) would increase with increasing N or I.
  • Figure 5-B shows four polarization curves that extended to 13 Nolts.
  • the velocities of the membrane relative to the work piece during these experiments were 19 cm sec, 30 cm/sec, 43 cm/sec, and 64 cm/sec, and the width of the area of contact measured in the direction of v was such that the corresponding amount of time the membrane would contact a single point on the substrate surface (the "dwell time") was about 29 msec, 18 msec, 13 msec, and 9 msec, respectively.
  • Removal rate is proportional to the ratio (I/n).
  • I the ratio of the current density
  • n increases with N for voltages >10N
  • the increase in n was proportionately smaller than the increase in I. Accordingly, under these conditions, removal rate increased with increasing N.
  • the membrane may become partially dehydrated, resulting in an increase in ohmic resistance. That is, while, at lower voltages, we believe a thicker low-conductivity fluid layer limits I by increasing the resistance proportional to the thickness of the water layer, we believe, at high N, the ohmic resistance increases when the rate of water consumption between the membrane and the substrate surface exceeds the rate of water supply.
  • N >10 N preferably N >14 N
  • polishing parameters that maximize I at N ⁇ 7 N can inhibit or limit I at N >10 N, for example at N >14 N.
  • the increase in n is proportionately lower than the increase in I, so the copper removal rate like the current density I increases with N. It is now easy to achieve the commercially desirable polishing rates of greater than 500 nm per minute, preferably greater than 700 nm/min, with relatively modest ratios of A c /A p , low tool pressure F z /A p , and low contact pressure F z /A c .
  • Another aspect of the invention is a process of membrane mediated electropolishing of a an area A p of a substrate, wherein the process includes: A) providing a substrate having a surface comprising copper, wherein at least a portion of the copper on said surface is electrically connected to the positive terminal of a substantially DC electrical power supply, and wherein the surface is at least partially wetted with a low-conductivity fluid comprising water and having a conductivity of less than about lxl 0 "4 S/cm; B) movably contacting the surface with a cathode half-cell comprising: 1. a cathode electrically connected to the negative terminal of the substantially DC electrical power supply; 2.
  • a cation-selective ion-conducting membrane having an interior side and an exterior side, wherein the membrane under the applied voltage allows more than half of the polished metal cations in the low-conductivity fluid to migrate through the membrane to the electrolyte, but allows less than 1 anion from the electrolyte to migrate to the low-conductivity fluid per 100 metal cations migrating from the low-conductivity fluid to the electrolyte, and wherein the exterior side is in contact with the wetted copper surface over a contact area A c ; and 3.
  • a conductive electrolyte having a conductivity of greater than about lxlO "2 S/cm in electrical contact with the cathode and with the interior side of the membrane, wherein the area A p over which the membrane movably contacts is greater than A c , and wherein the membrane exerts a force F z that is greater than 0; and C) supplying the low-conductivity fluid comprising water to at least to the surface areas of the substrate surrounding A 0 ; wherein the exterior surface of the membrane movably contacts the wetted surface of the substrate at a velocity greater than about 30 cm sec relative to the substrate surface, and wherein the voltage between the electrolyte and the metal is greater than 10 N.
  • Another aspect of the invention is a process of membrane mediated electropolishing of an area A p of a substrate, wherein the process includes: a) providing a cathode half-cell containing a cathode connected to the negative terminal of an electrical power supply and in contact with a conductive electrolyte composition, and a cation-selective, ion-conducting membrane situated so as to contact on a first side the conductive electrolyte solution and on the opposite side the surface of the substrate and a layer of low-conductivity de-ionized water disposed on the surface; b) providing a substrate having a surface with metallic copper disposed thereon, wherein said metallic copper is electrically connected to the positive terminal of the electrical power supply; c) supplying a low-conductivity fluid consisting essentially of de-ionized water to the surface of the substrate; d) movably contacting at least the surface A p of the substrate with the membrane, moving relative to and substantially parallel with the surface of the substrate at an average
  • Another aspect of the invention is a process of membrane mediated electropolishing of an area A p of a substrate, wherein the process includes: a) providing a substrate having a surface having a metal comprising copper which is electrically connected to the positive terminal of a substantially DC electrical power supply; b) providing a low-conductivity fluid comprising water and having a conductivity of less than about lxlO "4 S/cm to at least a part of the surface A p ; c) providing a cathode half-cell comprising: a cathode which is electrically connected to the negative terminal of a substantially DC electrical power supply, a cation-selective ion-conducting membrane having interior and exterior sides, wherein the membrane does not contact the cathode, and a conductive electrolyte having a conductivity greater than about lxl 0 "1 S/cm in electrical contact with the cathode and with the interior side of the membrane; and d) movably contacting the we
  • Another aspect of the invention is a process of membrane mediated electropolishing of an area A p of a substrate, wherein the process includes: a) providing a substrate having a surface having a metal comprising copper which is electrically connected to the positive terminal of a substantially DC electrical power supply; b) providing a low-conductivity fluid comprising water and having a conductivity of less than about 5xl0 "4 S/cm to at least apart of the surface A p ; c) providing a cathode half-cell comprising: a cathode which is electrically connected to the negative terminal of a substantially DC electrical power supply, a cation-selective ion-conducting membrane having interior and exterior sides, and a conductive electrolyte in electrical contact with the cathode and with the interior side of the membrane; and d) movably contacting the wetted surface of the subsfrate over the area A p with the exterior surface of the membrane while imposing a voltage N between the cathode and
  • the process in each embodiment is improved if the conductivity of the low- conductivity fluid is less than 50 ⁇ S/cm, preferably less than 20 ⁇ S/cm, for example between about 0.1 ⁇ S/cm and about 10 ⁇ S/cm.
  • the process in each embodiment is improved if the conductivity of the conductive electrolyte is greater than 0.1 S/cm, preferably greater than 0.3 S/cm.
  • the process of each embodiment is improved if the ratio A c /A p is 0.02 or higher, preferably 0.03 or high, for example greater than about 0.05.
  • the process of each embodiment is improved if the ratio A c /A p is 0.4 or less, preferably 0.2 or less, for example less than about 0.15.
  • the polishing half-cell may have an extent substantially greater than the total area of the substrate, the area A 0 is the area of the membrane contacting the surface A p at any one time.
  • the process described in each embodiment is a preferred process if the stoichiometric number n is sufficient to prevent precipitation of copper oxide, copper hydroxide, and copper carbonate on the substrate and in the low-conductivity fluid.
  • the stoichiometric number n representing the ratio between the current i and the rate of metal removal, is between about 3.9 and about 8.5 electrons/atom Cu, for example between about 4.1 and about 8 electrons/atom Cu electropolished from the substrate surface, alternately between about 4.5 and about 7.5 electrons/atom Cu electropolished from the substrate surface.
  • the voltage N is greater than 12 N, preferably greater than 14 N.
  • the slope dl/dN of a polarization curve at the voltage N is at least twice the average slope of the polarization curve from 3 Nolts to 7 Nolts.
  • the slope dl/dN at N has a positive second derivative.
  • the copper RR is equal to or greater than 500 nm min, preferably greater than about 700 nm/min, more preferably about 800 nm/min or greater, h each embodiment, it is advantageous if the current density is between about 0.8 A/cm 2 and 4 A/cm 2 , preferably between about 1 A cm 2 and 3.5 A cm 2 , for example between about 1.5
  • the slope dl/dN is greater than about 75 mA/cm 9 per volt, preferably greater than 100 mA/cm 9 per volt, more preferably greater than 150 mA cm 2 per volt.
  • the current density I increases when the interfacial velocity v is increased by 20%.
  • the voltage is greater than about 15 volts and if the interfacial velocity is sufficiently high so the polishing apparatus does not squeak.
  • the electro-polishing is performed at a maximum current density which is below 0.9I sq .
  • the voltage is greater than about 16 volts and if the interfacial velocity is greater than about 40 cm/sec.
  • the voltage is greater than about 12 volts and if the dwell time is less than about 25 milliseconds, preferably less than about 16 milliseconds, for example between about 6 milliseconds and about 14 milliseconds.
  • the current density I is greater than about 1.5 A/cm and if the velocity is such that the maximum uninterrupted time a portion of the substrate surface A p is covered by the membrane is less than about 0.2 seconds, for example wherein I is greater than greater than about 2 A/cm 2 and wherein the velocity is such that the maximum time a portion of the substrate surface is covered by the membrane is less than about 14 milliseconds.
  • the preferred substrate is a copper damascene wafer for the fabrication of integrated circuits.
  • the drag force F d of the membrane moving over the wetted substrate surface is less than one half, preferably less than one quarter, for example less than one sixth, the normal force F z exerted by the cathode half cell on the substrate.
  • I is greater than or equal to about 1 A/cm 2
  • n is greater than or equal to about 4
  • the substrate comprises low-k dielectric material, in which case F z /A c is preferably less than 5 psi, and P T is preferably less than 1 psi.
  • the substrate is membrane- mediated electropolished under conditions such that the interfacial velocity between the membrane and the substrate is between about 15 cm/sec and about 27 cm/sec, and such that the voltage N the current density I increases at a rate between about 50 mA/cm and 100 mA/cm 2 per volt.
  • Another aspect of the invention is a product by process, wherein the substrate (product) is a Cu-plated damascene wafer in the manufacture of integrated circuits.
  • the dielectric layer has substantially no loss of thickness from the Cu polishing process, e.g., less than 5 angstroms, alternately less than 2 angstroms, when the copper is polished from the substrate; 2) there are substantially no scratching and particulate defects on the polished surface after polishing, which affect component reliability; and 3) there is substantially no damage to, or alteration of, materials in the subsfrate, e.g., dielectric materials, metal layers, and the like, which are observed after the substrate is polished with a conventional chemical-mechanical polishing slurry.
  • FIGURES The descriptions herein can be more readily understood when read with reference to the following Figures:
  • Figure 1 is a schematic cross-section of a typical copper damascene wafer;
  • Figure 2 is a schematic illustration of the various physical dimensions governing planarization efficiency for EP in a convected elecfrolyte solution versus MMEP;
  • Figure 3 is a schematic cross-section of an MMEP cathode half-cell in contact with a work piece immersed in a low-conductivity solvent;
  • Figure 4 is a graph comparing the polishing efficiencies of MMEP and EP;
  • Figure 5 is a comparison of planarization efficiencies for EP and MMEP under steady-state conditions;
  • Figure 6 is a schematic illustration of the typical components of a spin-polishing apparatus used for MMEP on Cu coupons and Cu damascene wafers;
  • Figure 7 shows the variation in stoichiometric number (n, electrons per metal atom polished) with MMEP process parameters T 0ff and N
  • Figure 10-A shows a bottom view of a single -window cathode half-cell, and Figure 10-B shows a bottom view of a double- window cathode half-cell;
  • Figure 11-A shows a schematic illustration of MMEP operating parameters;
  • Figure 11-B shows an examples of a Cu coupon showing variations in elliptical contact areas obtained after static polishing at values of z ranging from 0 to -1.0 mm;
  • Figure 11-C shows a schematic of approximate ionic conductivity and thickness for phases intervening between the anode (substrate) and the cathode, where for each indicated layer thickness, the resulting ohmic resistance is calculated for a 1 cm 2 contact area ;
  • Figure 12- A shows an exemplary schematic illustration of the relationship between contact area A c and process area A p for a single-window half-cell;
  • Figure 12-B shows a graph of the effects of normal displacement z of an exemplary elliptical half-cell from the substrate, such as made the marks
  • P c in psi, is the contact pressure F z /A c ;
  • n in eVCu, is the number of electrons consumed by the process to polish an atom of copper from a substrate surface;
  • i in amps, is the current between the cathode and the substrate I, in amps/cm 2 , is the current density equal to i/A c ;
  • N in volts, is the voltage applied between the cathode and the subsfrate;
  • v in cm/sec, if the velocity of the membrane relative to the substrate surface;
  • dwell time in milliseconds, is the average width of the contact area A c measured in the direction of v, divided by the velocity v;
  • P T in psi, also called the tool pressure, is the force exerted by the cathode half cell normal to the wafer divided by the total area to be polished, equal to F z /A p ;
  • P T in psi, also called the tool
  • a c , in cm 2 is the contact area between the membrane and the substrate surface;
  • a p in cm 2 , is the total polished area of the substrate surface;
  • F z in gm, is the normal force exerted by the membrane on the substrate;
  • F d in gm, is the drag force exerted by the membrane on the substrate;
  • RR in nm/min, is the metal removal rate resulting from the membrane mediated electropolishing process, averaged over the entire polished area;
  • ⁇ m, in g is the mass lost from the substrate for any given period of polishing;
  • p is the density, which for Cu is 8.92 g/cm 3 ;
  • N m in cm /mole, is the molar volume, which
  • the invention comprises a method of membrane-mediated electropolishing of a work piece (8) having a surface comprising a metal, the method comprising or consisting essentially of: A) providing the work piece (8) covered with a low-conductivity fluid (9), wherein the work piece is in electrical contact with a first outlet of a substantially DC electrical power source having first and second power terminals (not shown); B) substantially and movably contacting the work piece covered with a low- conductivity fluid with a first side of an ion-conducting membrane (4), wherein a second side of the ion-conducting membrane contacts a conductive electrolyte composition (5); C) electrically contacting the conductive electrolyte composition (5) with a cathode (6), wherein the catho
  • the conductive electrolyte composition has a conductivity equal to or greater than about 5 mS/cm, preferably equal to or greater than about 30 mS/cm, and the low-conductivity fluid has a conductivity equal to or less than about 500 ⁇ S/cm, preferably equal to or less than about 100 ⁇ S/cm.
  • the conductive electrolyte composition does not contact the work piece.
  • the ion conductive membrane is substantially impermeable to the ions in the electrolyte, regardless of whether the conductive electrolyte is solubilized in a fluid similar to or the same as the low- conductivity fluid.
  • the voltage is varied for various metals, but the voltage range between IN and 50N is useful.
  • Current densities between 50 and 4000 mA/cm 2 preferably 200 to 2500 mA/cm 2 , are commercially useful.
  • n is advantageously maintained, e.g., n is greater than 3.5 moles of electrons per mole of copper electropolished when the current density is above about 500 mA/cm 2 .
  • n is between about 2 to about 3 moles of electrons per mole of copper electropolished when the current density is below about 400 mA/cm 2 .
  • ion exchange material means a material that captures metal ions, releasing other ions such other metals (other than the ions captured) or H + . Ion exchange material, as is known in the art, has a limited capacity to captures the metal ions, and can become saturated with the ions.
  • the ion-conducting membrane used in preferred embodiments of this invention allows selected ions to migrate therethrough, but does not substantially capture and hold these ions.
  • the quantity of metal ions removed in embodiments of the invention may be more than 1000 times greater than the exchange capacity of the membrane used.
  • an inlet and an outlet on the cathode half cell allowing the conductive electrolyte composition to be circulated through the cathode half cell, and optionally but advantageously a method and apparatus to de-gas the elecfrolyte composition, remove particulates from the electrolyte composition, add or remove selected constituents of the electrolyte composition which may be generated, exhausted or accumulated during electropohshing to adjust temperature, and/or control the pressure within the cathode half cell.
  • the method includes providing low-conductivity fluid at the interface of contact of the membrane and the work piece, wherein preferably the source of low-conductivity fluid supplies low-conductivity fluid both outside contact area between the membrane and the work piece, and within the contact area between the membrane and the work piece.
  • the ion-conducting membrane is a charge-specific ion-conducting membrane adapted to allow transport therethrough of positively charged ions or negatively charged ions, but not both.
  • the invention comprises a method of membrane- mediated electropolishing comprising or consisting essentially of: - providing a cathode half-cell comprising or consisting essentially of: a fully or partially enclosed volume, cavity or vessel; an electrolyte solution or gel which partially or essentially fills the enclosed volume, cavity or vessel; a cathode in contact with the elecfrolyte solution or gel; a connector electrically connecting the cathode to a DC power source; a charge-selective ion-conducting membrane which seals one surface of the enclosed volume, cavity or vessel in such a way that the internal surface of said membrane contacts the electrolyte solution or gel and the external surface is accessible to contact the metal work piece and a low-conductivity solvent or solution; and optionally a pump connected to an inlet and an outlet of the half cell and adapted to circulate the electrolyte composition between the enclosed volume, cavity or vessel and a second apparatus, wherein the second apparatus is adapted to change one or
  • the invention comprises a method of membrane- mediated elecfropolishing of a work piece having a surface comprising a metal, e.g., a semiconductor substrate, said method comprising or consisting essentially of: - providing the work piece, wherein the work piece comprises a metal-containing surface, and wherein said surface is electrically connected to a first terminal of an electrical power source and is substantially covered with a low-conductivity fluid; - providing a cathode half-cell comprising or consisting essentially of: a cathode electrically connected to a second te ⁇ ninal of an electrical power source and also electrically connected to a conductive electrolyte composition; a charge-selective ion-conducting membrane having first and second sides, wherein the second side contacts the electrolyte composition and the first side is adapted to movably contact the work piece and the low-conductivity solvent or solution; optionally a pump connected to an inlet and an outlet of the half cell and adapted to circulate
  • the source of low-conductivity fluid supplies low-conductivity fluid outside the contact area such that this fluid is carried into the contact area by translation of the contact area, or by contacting by creating new areas between non- contacting areas, and/or by diffusion of fluid from the periphery of the contact area.
  • This above embodiment also allows for a plurality of membranes and an accompanying plurality of electrolyte compositions within the half cell.
  • the invention comprises a method of membrane- mediated electropolishing of a work piece having a surface comprising a metal, e.g., a semiconductor substrate, said method comprising or consisting essentially of: - providing a cathode half-cell comprising or consisting essentially of: a cathode electrically connected to a terminal, e.g., the negative terminal, of an electrical power source and electrically connected to a half-cell fluid, wherein the half-cell fluid is adapted to carry electrical current between the cathode and a semipermeable membrane and to remove metallic ions from the membrane; and a semipermeable membrane having first and second sides, wherein the first side contacts the half-cell fluid and the second side is adapted to movably contact or be placed in very close proximity to a work piece and a low-conductivity fluid; - providing the work piece comprising a work piece surface comprising a metal, wherein said work piece surface is electrically connected to another terminal, e.g.,
  • half cell- fluid for this last particular embodiment encompasses for example liquids, suspensions, gels, solutions, emulsions, and specifically includes low viscosity and higher viscosity compositions, and any mixtues thereof.
  • semipermeable means the membrane is permeable to metal-containing ions generated from the work piece but is substantially impermeable to at least one component of the half-cell fluid.
  • the half-cell fluid has a higher conductivity than does the low-conductivity fluid, for example at least a factor of 100 greater conductivity.
  • One advantage of MMEP over EP is its superior ability to reduce roughness and to planarize a surface.
  • MMEP planarization efficiency of MMEP is intrinsically much greater than that of EP under typical conditions with convected electrolyte, and is also much greater than for an ideal EP process with no convection.
  • convection is essential for most applications of EP because in the absence of convection, polishing is too slow and non- uniform to be of practical value.
  • MMEP is capable of planarizing Cu damascene wafers featuring humps and dishing in the plated topography without the use of additional means such as masking, or mechanical abrasion.
  • MMEP provides the advantage over EP and electromechanical polishing processes in that no strong acid electrolytes, metal salts, or abrasive particle contact the wafer.
  • MMEP is useful for polishing and planarizing work pieces of various compositions, including both pure metals and alloys. Accordingly MMEP has utility for many of the same metal finishing applications currently addressed by conventional EP.
  • MMEP can provide several practical advantages over EP for those applications and also for other applications such as planarization of copper damascene wafers where conventional EP suffers from intrinsic limitations as previously discussed. For example, in MMEP, anodic dissolution occurs only at locations in contact with, or in close proximity to the cathode half-cell membrane so that polishing can be easily restricted to selected portions of the work piece.
  • MMEP also requires only a small volume of elecfrolyte sufficient to fill the cell, and this may be much smaller than the volume required to polish an equivalent area of work piece by coating or immersion in conventional EP. Moreover, the work piece in MMEP never directly contacts the electrolyte solution, and essentially all metal ions produced by anodic dissolution are transferred across the membrane and into the half cell. Thus the work piece remains free of contamination by toxic and/or corrosive chemicals typical of EP electrolytes. Under suitable operating conditions, virtually all of the metal ions generated in MMEP are captured by the cathode half-cell and plated onto the cathode, so that the amount of metal salts in the electrolyte remains constant.
  • the invention also includes the apparatus adapted to perform the invention, the requirements of which are easily determined from the method.
  • this invention also provides an apparatus for use in membrane-mediated electropolishing of a metal work piece, comprising: - an electric, e.g., substantially DC, power source; - a means for electrically connecting the work piece to the positive terminal of the substantially DC electric power source, e.g., a connector; - an elecfrolyte composition, e.g., solution or gel; - a low-conductivity fluid, e.g., solvent or solution, which is in contact with the membrane and the work piece; - an ion-conducting, e.g., a charge-selective ion-conducting, membrane which seals one surface of the enclosed volume, cavity or vessel in such a way that the internal
  • the above apparatus can optionally include a pump connected to an inlet and an outlet of the half cell and adapted to circulate the electrolyte composition between the enclosed volume, cavity or vessel and a second apparatus, wherein the second apparatus is adapted to change one or more of the temperature of the electrolyte composition, the composition of the electrolyte composition, or the pressure of the electrolyte composition within the enclosed volume, cavity or vessel.
  • the above apparatus can optionally include a low-conductivity fluid source adapted to provide low-conductivity fluid between the membrane and the work piece. Such a source can be steady state, pulsed, or some combination thereof.
  • Cathode Half Cell This invention also provides a cathode half-cell for use in membrane-mediated elecfropolishing.
  • the half cell comprises: - a fully or partially enclosed volume, cavity or vessel (5); - an electrolyte solution or gel which partially or substantially fills the enclosed volume, cavity or vessel (not shown); - a cathode (6) which is in contact with the elecfrolyte solution or gel; - a charge-selective ion-conducting membrane (4) which seals at least one surface of the enclosed volume, cavity or vessel, wherein the internal surface of the membrane is in contact with the electrolyte solution or gel; and - optionally inlet and outlet (7) for circulating the electrolyte solution or gel.
  • the base may comprise a backplate (1), sides (3), a compression element (2) adapted to compress, hold, and seal the membrane (4) against the sides (3).
  • the work piece (8) is substantially covered with low-conductivity fluid (9).
  • Another aspect of the invention relates to a new type of EP process whereby, in one embodiment, the work piece is physically separated from the electrolyte and cathode by a charge-selective ion-conducting membrane, wherein the membrane is essentially impermeable to the electrolyte solutes (and acids) but permeable to the ions produced by anodic oxidation of the work piece.
  • a layer of low-conductivity fluid also separates the work piece from the membrane.
  • MMEP membrane-mediated electropolishing process
  • a cathode half-cell which can be a fully or partially enclosed volume, cavity or vessel.
  • the half-cell contains a cathode in contact with an electrolyte solution or gel, and is sealed on at least one surface with a charge-selective ion-conducting membrane.
  • the electrolyte in the half-cell is maintained at a hydrostatic pressure greater than ambient atmospheric pressure, and the membrane is sufficiently flexible to expand under the influence of this pressure to establish a convex external surface (a "bulge” or "blister") extending beyond adjacent surfaces of the half-cell to contact work piece.
  • a source of DC electrical power is connected between the work piece (which functions as the anode) and the cathode in the cell (which functions as the cathode). Polishing is accomplished when a portion of the external surface of the ion-conducting membrane is brought into contact with a portion of the work piece otherwise covered by the low-conductivity solvent, and this area of contact is moved across the surface of the work piece. It is to be understood that in many applications of this process, the work piece can be held stationary and the membrane moved across its surface, or the half-cell can be held stationary and the work piece moved, or both the work piece and the half-cell can be in motion, provided that the area of contact is not static. Movement may consist of rubbing, sliding, or rolling the surface of one component over that of the other component.
  • Low-conductivity Fluid hi every embodiment of the invention, a low- conductivity fluid is used to contact, e.g., cover, the work piece.
  • the low-conductivity fluid serves to solvate the metal ions and facilitate their transport through the membrane, and also to limit the anodic dissolution reaction to areas of the work piece in contact with, or in close proximity to, the membrane.
  • the electrical conductivity of the low-conductivity fluid advantageously is low, for example less than 1200 ⁇ S/cm, preferably less than 500 ⁇ S/cm, more preferably less than 200 ⁇ S/cm.
  • the planarization efficiency of an MMEP process increases as the conductivity of the low-conductivity fluid decreases.
  • the electrical conductivity of the low-conductivity fluid can be as low as 0.1 ⁇ S/cm, e.g., or the conductivity of ulfrapure water.
  • the electrical conductivity of the low-conductivity fluid is between about 0.5 ⁇ S/cm and about 150 ⁇ S/cm, for example between about l ⁇ S/cm and about lO ⁇ S/cm if the ⁇ is greater than 10 microns, and between about 1 l ⁇ S/cm and about 50 ⁇ S/cm if the ⁇ is between 1 and 10 microns. While very low-conductivity fluid is useful for embodiments where ⁇ is less thanlO microns (even less than 1 micron), such low conductivity is not essential to achieve commercially acceptable planarization efficiency.
  • the low-conductivity fluids have a conductivity of about 200 ⁇ S/cm or lower, for example between about 50 ⁇ S/cm and 200 ⁇ S/cm, alternatively between about 5 ⁇ S/cm and 50 ⁇ S/cm.
  • the low-conductivity fluid resistance is greater than about 0.4 M-ohm-cm.
  • the low-conductivity fluid resistance may be greater than 2 M-ohm-cm, e.g., between about 6 to about 18 M-ohm-cm.
  • the low-conductivity fluid must have a conductivity sufficient to conduct the required current density, but the thickness of the layer of low-conductivity fluid between the membrane and the work piece is kept low, e.g., about 0.01 to about 2 microns, so that the very low-conductivity fluids are operative. For example, at a conductivity of 10 ⁇ S/cm, areas more distant from the membrane than 100 microns would be effectively "protected" by a kilo-ohm of resistance relative to areas that are in contact. This is sufficient to suppress the rate of oxidation of Cu at ⁇ 10 N over potential. Generally, the conductivity of the low-conductivity fluid can change substantially if dissolved carbon dioxide is present.
  • the low- conductivity fluid be de-gassed and/or purged with an inert gas such as nitrogen. Maintaining a nitrogen atmosphere in the MMEP apparatus prevents air and other undesirable gases from entering therein, and at the same time prevents undesirable contaminant particulates from entering the polisher.
  • Solvating Capacity Of The Low-conductivity Fluid To satisfy the solubility condition, the solvent or some component of the solution must be capable of solvating or otherwise coordinating the metal ions produced at the anode to create an ionic complex that is soluble and mobile within the membrane. The nature of the anode reaction may also be influenced by the composition of the low-conductivity solvent in other ways.
  • the viscosity of the low-conductivity fluid is between about 0.3 centipoise (cp) to about 100 cp, for example between about 0.8 cp to about 3 cp.
  • the low-conductivity fluid comprises, consists essentially of, or consists of water, acetonitrile, ethylene carbonate, propylene carbonate, glycol, ethylene glycol, propylene glycol, diethlyene glycol, or mixtures thereof.
  • the low-conductivity fluid used in MMEP that contacts both the anode (work piece) and the external surface of the membrane can be a composition comprising one or more of water, selected solvents, and less than 300 ppm surfactants and/or viscosifying agents.
  • the surfactants can include "passivating" surfactants or surface-active agents. Such agents may tend to deposit onto the surface of the work piece, inhibiting electropolishing at these locations, e.g., in depressions within the surface.
  • Such a surfactant or surface active agent must be compatible with contacting the membrane, e.g., must not plug the membrane, and should be readily swept away by contact with the membrane.
  • the low-conductivity fluid can be a gel.
  • the pH is determinative of the metal-ion solvating capacity of the low- conductivity fluid.
  • Many metals ions are soluble in an acidic environment, so in another embodiment the low-conductivity fluid further comprises acid sufficient to maintain for example apH of between 2 and 6.5, for example about 3 to about 6 or about 4 to about 5.
  • a pH of between 4 and 6.9 is useful, for example between 6 and 6.5.
  • Acids can include for example phosphoric acid, pyrophosphoric acid, carboxylic acids, or mixtures thereof.
  • sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, pyrophosphoric acid, carboxylic acids, or mixtures thereof may be the acid of choice.
  • the metal ions electropolished from the work piece should be stable in the acid, e.g., not form precipitates.
  • Other acids are specified for selected metals as discussed infra.
  • the low-conductivity fluid further comprises base sufficient to maintain for example a pH of between about 7.1 to about 12, for example between about 7.5 to about 9.
  • Useful bases include ammonium hydroxide, halide hydroxides, ammonium sulfate, amines or salts of carboxylic acids.
  • the low-conductivity fluid further comprises a buffer sufficient to maintain a pH during electropolishing of within about 0.2 pH units of the initial pH.
  • the low-conductivity fluid further comprises a chelator.
  • the low- conductivity fluid comprises, consists essentially of, or consists of water
  • this low-conductivity fluid further comprises acid sufficient to maintain for example a pH of between about 3.9 to about 6.5, for example between about 4.5 to about 6.
  • Acids can include for example phosphoric acid, pyrophosphoric acid, carboxylic acids, or mixtures thereof.
  • this low-conductivity fluid further comprises a buffer sufficient to maintain a pH during elecfropolishing of within about 0.2 pH units of the initial pH.
  • the low-conductivity fluid comprises, consists essentially of, or consists of, water and at least one of acetonitrile, proprionitrile, butyronitrile, glycol, an alcohol having from 1 to 8 carbon atoms, ethylene glycol, propylene glycol, diethlyene glycol, proplylene glycol monomethyl ether, propylene carbonate, ethylene carbonate, and dimethylformamide.
  • this low-conductivity fluid further comprises acid sufficient to maintain for example a pH of between about 3.9 to about 6.5, for example between about 4.5 to about 6.
  • Acids can include for example phosphoric acid, pyrophosphoric acid, carboxylic acids, or mixtures thereof. Other acids are specified for selected metals as discussed infra.
  • this low- conductivity fluid further comprises a buffer sufficient to maintain a pH during electropolishing of within about 0.2 pH units of the initial pH.
  • this low-conductivity fluid further comprises a chelator.
  • Specific examples of preferred water-based low-conductivity fluids include 1) pure, very low conductivity water, 2) pure water having less than about 0.001 weight percent acid such as sulfuric acid, phosphoric acid, nitric acid, organic acid, or combination thereof, 3) pure, very low conductivity water having from 0.1% to 70% acetonitrile, and optionally having less than about 0.001 weight percent acid such as sulfuric acid, phosphoric acid, nitric acid, organic acid, or combination thereof.
  • the low- conductivity fluid comprises, consists essentially of, or consists of water and acetonitrile. In a more preferred embodiment the amount of acetonitrile is sufficient to give an electropolishing efficiency of between one metal atom, e.g., one copper atom, per 1 to 1.99 electrons of current flow.
  • the low-conductivity fluid comprises, consists essentially of, or consists of water and glycols, wherein the term glycols includes for example glycol, ethylene glycol, propylene glycol, diethlyene glycol, proplylene glycol monomethyl ether, or mixtures thereof.
  • the low-conductivity fluid comprises, consists essentially of, or consists of water and a nitrile-containing solvent such acetonitrile, a copper-coordinating solvent such as ethanolamine or ethylene diamine, ethylene carbonate, propylene carbonate, In an alternate embodiment this low-conductivity fluid further comprises a chelator.
  • chelators can optionally be added.
  • the low-conductivity fluid is substantially free of chelators, except for acetonitrile or other substantially non-polar complexing agents, or small quantities, e.g., 0.005% to 0.2%, of one or more of dicarboxylic acids or citric acid.
  • the chelators and/or chelated metal complexes have an electrical charge which helps the complex migrate toward the membrane, so long as the overall conductivity constraints of the low-conductivity fluid are met.
  • the chelator-metal complex is advantageously able to either release the metal ion to the membrane, or must be able to pass through the membrane.
  • Chelators include those chelators normally found in the art, e.g., EDTA, DTP A, citric acid, di-functional organic acids including for example di- or multi- hydroxybenzene-type compounds, e.g., such as catechol, butylated hydroxytoluene ("BHT"), and the like; dicarboxylic acids such as oxalic acid, malonic acid, succinic acid, glutamic acid, adipic acid, maleic acid, fumaric acid, phthalic acid, isophthalic acid, terephthalic acid and the like; tricarboxylic acids such as citric acid, aconitic acid, trimellitic acid and the like; hydoxycarboxylic acids such as glycolic acid, gallic acid, lactic acid, 2-hydroxybutyric acid, tartaric acid, malic acid, salicylic acid, and the like; ketocarboxylic acids, such as acetoacetic acid, ketoglutaric acid and the like; aminocar
  • any acidic chelator can be present partially or completely as a salt, e.g., an ammonium salt, hi some embodiments the chelator comprises one or more of salicylic acid, gallic acid, glycolic acid, citric acid, or mixture thereof.
  • viscosifying agent which are typically polymeric, or viscosity reducers such for example 0.01% to 20% by .weight of a small quantity a low molecular weight alcohol, can also be included.
  • Non-Water-Based Low-conductivity fluid contemplates the use of low-conductivity fluids that are substantially free of water. Generally, it is necessary for some component in the low-conductivity fluid to coordinate with and solvate the metal ions.
  • the low-conductivity fluid comprises, consists essentially of, or consists of acetonitrile, proprionitrile, butyronitrile, or mixture thereof.
  • the low-conductivity fluid comprises, consists essentially of, or consists of acetonitrile, proprionitrile, and/or butyronitrile, one or more alkanolamines, such as ethanolamine, diefhanolamine, triethanolamine, isopropanolamine, ethylene diamine, or mixtures thereof, hi another embodiment the low-conductivity fluid can further comprise ammonia.
  • alkanolamines such as ethanolamine, diefhanolamine, triethanolamine, isopropanolamine, ethylene diamine, or mixtures thereof
  • the low-conductivity fluid can further comprise ammonia.
  • the value of n may be less than 2 electrons per metal atom, though efficiencies between about 2.5 to 4.5 electrons per metal ion generated is more common.
  • These complexing solvents can stabilize a single-charged copper complex, i.e., Cu(I).
  • the amount of these nitrogen-containing solvents in the low- conductivity fluid is sufficient to give a electropolishing efficiency of between one copper atom per 1 to 1.99 electrons of current flow.
  • the normally doubly charged metal ions e.g., copper
  • the low-conductivity fluid and the electrolyte solvent each independently comprise, consist essentially of, or consist of acetonitrile, proprionitrile, butyronitrile, or mixture thereof, wherein the amount of acetonitrile is sufficient to coordinate and stabilize metal single-charged ions, e.g., Cu +1 , give a electropolishing efficiency of between one metal atom, e.g., one copper atom, per 1 to 1.99 electrons of current flow, hi an experiment using pure acetonitrile as the low- conductivity fluid and a mixture of acetonitrile and tetramethylammonium hexafluorophosphate as the electrolyte, the electrolyte being separated from the low- conductivity fluid by a Nafion ® membrane, the unit passed current to the work piece, electropolished metal therefrom, and solvated the polished metal.
  • the low-conductivity fluid comprises, consists essentially of, or consists of an alcohol having from 1 to 8 carbon atoms and at least one of acetonitrile, glycol, ethylene glycol, propylene glycol, diethlyene glycol, proplylene glycol monomethyl ether, propylene carbonate, ethylene carbonate, N,N-dimethlyacetamide, N- methylpyrolidone and dimethylformamide.
  • the low-conductivity fluid comprises, consists essentially of, or consists of a glycol and at least one of acetonitrile, proplylene glycol monomethyl ether, propylene carbonate, ethylene carbonate, and dimethylformamide. In one embodiment this low-conductivity fluid comprises, consists essentially of, or consists of a glycol and acetonitrile. In one embodiment the low-conductivity fluid comprises, consists essentially of, or consists of propylene carbonate, ethylene carbonate, or mixture thereof.
  • the low-conductivity fluid comprises, consists essentially of, or consists of a non-polar organic solvent and at least one of acetonitrile, glycol, ethylene glycol, propylene glycol, diethlyene glycol, proplylene glycol monomethyl ether, propylene carbonate, ethylene carbonate, and dimethylformamide.
  • the low-conductivity fluid comprises, consists essentially of, or consists of a polar organic solvent.
  • an amount of any component is a function of the solvating capacity, lubricating qualities, and conductivity of that component.
  • polar organic solvents include substituted sulfoxides including dimethyl sulfoxide; glycols including glycol, ethylene glycol, diethlyene glycol, triethlyene glycol, ethylene glycol alkyl ether, diethylene glycol alkyl ether, triethylene glycol alkyl ether, propylene glycol, dipropylene glycol, propylene-ethylene glycols, and propylene glycol alkyl ether, e.g., proplylene glycol monomethyl ether, N-substituted pyrrolidone such as N-methyl-2- ⁇ yrrolidone (NMP), sulfolanes, substituted sulfones including 2,4-dimethylsulfone, substituted amides including dimethylacetamide, or any combination thereof.
  • NMP N-substituted pyrrolidone
  • NMP N-methyl-2- ⁇ yrrolidone
  • sulfolanes substituted sul
  • the low- conductivity fluid comprises a polar organic solvent and at least one of acetonitrile, propylene carbonate, ethylene carbonate, and dimethylformamide.
  • the low-conductivity fluid comprises, consists essentially of, or consists of a fluid that is substantially immiscible in the electrolyte.
  • the low-conductivity fluid may comprise a non-polar organic solvent, or a solvent with a substantial non-polar character, such as an alkyl-benzene, or an alcohol with greater than 8 carbon atoms.
  • this organic-based low-conductivity fluid has a sufficient capacity to solubilize the metal ions.
  • this low-conductivity fluid further comprises one or more compounds to coordinate with the metal ions, wherein said compound or compounds are present in an amount sufficient to solubilize the metal ions removed from the work piece.
  • surfactants such as alkyl sulfonates where the alkyl group comprises for example more than six carbon atoms.
  • Acetonitrile, proprionitrile, butyronitrile, and/or any R-nitrile (where R is a CI to C9 alkyl or aryl moiety) compatible with the membrane and the work piece are another example.
  • Other chelators soluble in the organic-based low-conductivity fluid can be included.
  • An advantage of this embodiment is the membrane can more readily keep the electrolyte within the cathode half-cell, as the electrolyte can be selected to have negligible solubility in the organic-based low-conductivity fluid.
  • Another advantage of the organic-based low-conductivity fluid is that such fluids can have resistivities greater than even relatively pure water.
  • the low-conductivity fluid and the elecfrolyte solvent can be only slightly miscible one with another.
  • 2,4-dimethylsulfolane is only slightly miscible with water and for such an embodiment, the electrolyte solvent can comprise or consist of 2,4-dimethylsulfolane and the low-conductivity fluid can comprise or consist essentially of water, or the electrolyte solvent can comprise or consist of water and the low-conductivity fluid can comprise or consist essentially of 2,4- dimethylsulfolane.
  • butyronitrile is only slightly miscible with water.
  • the combination can comprise 2 or more nitriles, e.g., acetonitrile in one fluid and butyronitrile in the other fluid, for example.
  • chelators can optionally be added.
  • the low-conductivity fluid can be substantially free of chelators.
  • the chelators and/or chelated metal complexes have an electrical charge which helps the complex migrate toward the membrane, so long as the overall conductivity constraints of the low- conductivity fluid are met.
  • the chelator-metal complex is advantageously able to either release the metal ion to the membrane, or must be able to pass through the membrane.
  • Chelators include those chelators normally found in the art, e.g., EDTA, DPT A, citric acid, di-functional organic acids including for example di- or multi- hydroxybenzene-type compounds, e.g., such as catechol, butylated hydroxytoluene ("BHT"), and the like; dicarboxylic acids such as oxalic acid, malonic acid, succinic acid, glutamic acid, adipic acid, maleic acid, fumaric acid, phthalic acid, isophthalic acid, terephthalic acid and the like; tricarboxylic acids such as citric acid, aconitic acid, trimellitic acid and the like; hydoxycarboxylic acids such as glycolic acid, gallic acid, lactic acid, 2-hydroxybuteric acid, tartaric acid, malic
  • the chelator- metal complex be able to pass through the membrane, or that the chelator readily release the metal ion to allow the metal ion to pass through the membrane.
  • any acidic chelator can be present partially or completely as a salt, e.g., an ammonium salt.
  • the chelator comprises one or more of salicylic acid, gallic acid, glycolic acid, citric acid, or mixture thereof.
  • between about 0.001% to 0.1% of viscosifying agents, which are typically polymeric, can be added.
  • a low molecular acid may be added, for example.
  • a surfactant can also be included at for example 0.001% to 0.03% by weight.
  • surfactants which coat and passivate the surface of the work piece not contacted by the membrane.
  • the application generally discusses the embodiment including the electropolishing of copper, which is advantageously (but not necessarily) done in an acidic environment.
  • cathode is used universally herein to describe the electrode residing within the half cell, called the "cathode half cell.”
  • the cathode can be made from any electrically conductive material that is chemically stable in the electrolyte.
  • the cathode comprises the metal on the anode work piece.
  • a copper cathode is useful for polishing copper, as copper ions reduced to metal at the cathode will typically stably plate onto such cathode.
  • the cathode is a material which has poor adherence to the plated material can be used.
  • the metal ions removed from the work piece are reduced to metal at the cathode, but this metal readily falls off the cathode and is advantageously carried from the half-cell by the electrolyte.
  • ⁇ on-plating cathode can comprise tantalum, titanium, 304 stainless, 306 stainless, chromium, or other non- depositing metal, so that the metal ions produced from the anode will not adhere to the cathode.
  • the shape of the cathode is a plate extending in a plane roughly parallel to the membrane, hi preferred embodiments, the cathode is planar and comprises a plate or screen, and the cathode extends over substantially an entire area equal to, or greater than, and laterally displaced from the "membrane contact area", defined as the area in which the membrane substantially contacts or is kept in close proximity to the work piece. For highly conductive elecfrolytes, the shape of the cathode is less important.
  • one cathode embodiment comprises a flat plane extending over the "membrane contact area", and is disposed less than 300 microns from the membrane, e.g., less than about 100 microns or between about 10 and about 50 microns from the membrane, but does not contact the membrane.
  • the electrolyte has high conductivity, then the resistance between the cathode and the membrane is usually negligible.
  • the electrolyte is itself of lower conductivity, for example having a resistance greater than about 0.1 M-ohm-cm, then the shape and the distance between the work piece and the cathode becomes critical.
  • the cathode comprises a flat plane extending over the "membrane contact area", and is disposed less than 10 microns from the membrane, e.g., less than about 5 microns or between about 1 and about 3 microns from the membrane. In such embodiments, maintaining the distance at a stable amount becomes important, and non-conducting support structures extending from cathode to the membrane may be employed.
  • the cathode comprises holes therethrough adapted to allow electrolyte flow from the back of the cathode and through said holes to the volume between the cathode and the membrane.
  • the cathode may comprise channels adapted to facilitate flow of electrolyte across the surface of the cathode.
  • Electrolyte i every embodiment of the invention, an electrolyte solution is disposed between, and conducts current between, the cathode and the interior surface of the membrane.
  • the elecfrolyte comprises an electrolyte solvent, and one or more additives (typically additives which dissociate to form charged species) including for example acids and/or buffers, salts, e.g., metal salts, ionized compounds, chelators, surfactants, and/or viscosifiers.
  • cathode and electrolyte compositions For many typical anode compositions suitable for use in MMEP, corresponding suitable choices for cathode and electrolyte compositions can be found in Electroplating Engineering Handbook, 4 th Edition, pp. 100-120, by D.E. Ward, L.J. Durney, Ed., NanNostrand Co., NY, 1984.
  • the electrolyte is chosen to provide high solubility for the metal ions or coordinated metal ions produced by oxidation of the anode.
  • the capacity of the elecfrolyte to solubilize the metal ion/coordinated metal ion will depend on the targeted polishing rates and the effective migration rates from the membrane to the cathode.
  • the electrolyte carries current between the cathode and the membrane.
  • Preferred electrolyte compositions have sufficiently high conductivity to carry current densities up to several hundred mA/cm 2 without introducing significant voltage drop or heating.
  • Conductivities of electrolytes is advantageously at least 30 mS/cm, and conductivities of at least 100 mS/cm, for example at least about 1000 mS/cm, alternatively between about 1000 mS/cm and about 10000 mS/cm, are preferred.
  • the voltage drop from the cathode to the membrane is substantially negligible, greatly simplifying the cathode half-cell design.
  • the viscosity of the electrolyte is between about 0.4 centipoise (cp) to about 100 cp, for example between about 0.8 cp to about 3 cp.
  • suitable electrolytes include metal salts of poly(acrylic acid), poly(methylacrylic acid), poly(styrene sulfonic acid), lightly cross-linked derivatives and copolymers thereof.
  • Light cross-linking refer to a low percentage of the monomers of one backbone linking to other polymer backbones or monomers with the same backbone for example from about 0.001% to about 5%, alternatively from about 0.001% to about 1 %, alternatively from about 0.01% to about 2%.
  • the polymers are advantageously ay least 30% neutralized, for example at least 60% neutralized, with metal, e.g., with ions of the metal being polished.
  • Acid- Water Electrolyte Compositions The most common standard electropolishing electrolytes are concentrated aqueous solutions of strong acids. Examples 1-5 illustrate the use of various strong acid electrolytes for MMEP.
  • the cathode can be made from any electrically conductive material that is chemically stable in the electrolyte. Chloride ion has been found to be effective in maintaining the solubility of Sn +2 and Al +3 ; a few representative examples of electrolytes that can be used for MMEP are listed in Table 2.
  • the invention encompasses electrolytes made with sufficient, i.e., at least 3%, preferably at least 10%, e.g. at least 20%, of an acid, and is substantially free of added salts.
  • the electrolyte consists essentially of water and an acid such as phosphoric acid, nitric acid, sulfuric acid, strong organic acids, e.g., acetic acid, hydrochloric acid, or mixture thereof, hi another embodiment, the electrolyte consists essentially phosphoric acid, nitric acid, sulfuric acid, strong organic acids, e.g., acetic acid, hydrochloric acid, or mixture thereof.
  • Electrolysis with low metal ion concentration will result in hydrogen production at the cathode. This dissolved hydrogen can react with metal ions to precipitate metal particles.
  • the electrolyte composition in the form of soluble metal salts, metal complexes, metal-neutralized hydrophilic polymers such as metal-neutralized polyacrylates, or combinations thereof. It is preferred that the elecfrolyte comprise between 0.01M and 2.5 M of metal ions. More metal ions is not deleterious but may not be necessary. While the metal concentration may exceed 2.5 M, it need not all be readily reduced at the cathode. For example, some metal in a metal-neutralized hydrophilic polymer may not be readily reduced at the cathode.
  • Simple dissociated salts are the most readily reducible metal ions, and therefore it is preferred that at least 0.1 M of metal ions, preferably at least 0.5 M of metal ions, are substantially dissociated salts in the electrolyte that are easily reduced at the cathode. A high concentration of metal ions is maintained in the electrolyte to suppress hydrogen generation and prevent formation of metal particles.
  • Solvents For The Electrolyte Composition The various electrolyte solvents can not, with the exception of a few strong acids, support the desired current flow.
  • the electrolyte is typically a composition comprising at least a solvent, e.g., water, an acid, or mixture thereof, and one or more ionic compounds dissolved therein the increase the conductivity of the electrolyte.
  • the salts and acids do not react with the metal ions entering the electrolyte solution through the membrane.
  • the electrolyte solvent typically comprises or consists essentially of water, an acid as listed above, a polar organic solvent, a solvent such acetonitrile, ethylene carbonate, propylene carbonate, and/or a glycol, or a mixture thereof.
  • the electrolyte composition comprises water as a solvent.
  • molten salt containing no solvent is used instead of the solvent.
  • the various components of the electrolyte composition must be compatible, though compatibility of various combinations is usually predictable by of one of ordinary skill in the art with little or no experimentation.
  • the cathode can be made from any electrically conductive material that is chemically stable in the electrolyte.
  • one or more non-acid, ionizable compounds are added to the electrolyte composition.
  • the most common embodiments of this invention include concentrated aqueous solutions of strong acids in addition to added salts.
  • Examples 1-5 illustrate the use of various strong acid electrolytes for MMEP. Chloride ion has been found to be effective in maintaining the solubility of Sn +2 and Al +3 ; a few representative examples of electrolytes that can be used for MMEP are listed in Table 2.
  • One class of charged compounds typically added are acids, preferably wherein the acids have high dissociation constants for at least one hydrogen to provide substantially dissociated acid.
  • Acids can include for example phosphoric acid, pyrophosphoric acid, nitric, hydrochloric, sulfuric, carboxylic acids, or mixtures thereof, an alternate embodiment the charged compounds further comprises a buffer sufficient to maintain a pH during electropolishing of within about 0.1 pH units of the initial pH.
  • the invention is applicable to a wide variety of metals, and some metals, as is known in the art, require a basic pH to be stably electropolished and solvated by the low-conductivity fluid and/or elecfrolyte composition.
  • a base may be used.
  • Useful bases include ammonium salts, as well as soluble halide hydroxides.
  • Useful pH ranges include 7.5-10.5 for the low-conductivity fluid and 8 to greater than 14 for the electrolyte.
  • the solvent of the electrolyte composition is substantially the same as the solvent in the low-conductivity fluid.
  • the membrane to be at least semipermeable to the solvent/low-conductivity fluid, without creating undesirable concentration gradients and without being permeable to the ionized portion of the electrolyte composition.
  • This embodiment excludes, obviously, those cases where the electrolyte solvent comprised high concentrations of acid, as the conductivity of such a liquid is too high for use as a low-conductivity fluid.
  • the following embodiments can be used when the electrolyte solvent is the same as the low-conductivity fluid, and of course can also be used when the low-conductivity fluid is different than the electrolyte solvent.
  • the electrolyte solvent comprises or consists essentially of water, acetonitrile, ethylene carbonate, propylene carbonate, glycol, ethylene glycol, propylene glycol, diethlyene glycol, or mixtures thereof.
  • there can be a small amount of acid e.g., an amount sufficient to give pH 2 to pH 7.
  • MMEP polishing of Cu in water requires an acidic environment at the surface of the anode because at pH >4 Cu +2 precipitates from water as CuO.
  • the electrolyte solvent comprises, consists essentially of, or consists of water and at least one of acetonitrile, a glycol, an alcohol having from 1 to 8 carbon atoms, ethylene glycol, propylene glycol, diethlyene glycol, proplylene glycol monomethyl ether, propylene carbonate, ethylene carbonate, and dimethylformamide, and again, optionally, a small amount of acid, e.g., an amount sufficient to give pH 2 to pH 7.
  • electrolyte solvent includes water, acetonitrile, proprionitrile, butyronitrile,or mixture thereof.
  • the electrolyte solvent comprises, consists essentially of, or consists of water and acetonitrile.
  • the low-conductivity fluid and the electrolyte solvent each independently comprise, consist essentially of, or consist of acetonitrile, wherein the amount of acetonitrile is sufficient to give a electropolishing efficiency of between one metal atom, e.g., one copper atom, per 1 to 1.9 electrons of current flow.
  • the elecfrolyte solvent comprises or consists essentially of a polar organic solvent.
  • polar organic solvents include substituted sulfoxides including dimethyl sulfoxide; glycols including glycol, ethylene glycol, diethlyene glycol, triethlyene glycol, ethylene glycol alkyl ether, diethylene glycol alkyl ether, triethylene glycol alkyl ether, propylene glycol, dipropylene glycol, propylene-ethylene glycols, and propylene glycol alkyl ether, e.g., proplylene glycol monomethyl ether, N- substituted pyrrolidone such as N-methyl-2-pyrrolidone (NMP), sulfolanes, substituted sulfones including 2,4-dimethylsulfone, substituted amides including dimethylacetamide, or any combination thereof.
  • NMP N- substituted pyrrolidone
  • NMP N-methyl-2-pyrrolidone
  • sulfolanes substituted sulfones including 2,4-dimethyl
  • the electrolyte solvent comprises a polar organic solvent and at least one of acetonitrile, proprionitrile, butyronitrile, or any compatible R-nitrile where R is a C 3 to C 9 alkyl or aryl group, propylene carbonate, ethylene carbonate, and dimethylformamide.
  • a solvent of pure acetonitrile was found to work when a soluble ionic species (complex-able metal ions and/or other ions such as tetramethylammom ' um hexafluorophosphate or the like) were added thereto to achieve the desired conductivity.
  • the low-conductivity fluid and the electrolyte solvent are selected so that they are substantially immiscible.
  • 2,4-dimethylsulfolane is only slightly miscible with water, and for such a combination, the electrolyte solvent can comprise or consist of 2,4-dimethylsulfolane and the low-conductivity fluid can comprise or consist essentially of water, or alternatively the electrolyte solvent can comprise or consist of water and the low-conductivity fluid can comprise or consist essentially of 2,4-dimethylsulfolane.
  • Salts for The Electrolyte Composition The various elecfrolyte solvents cannot, with the exception of a few strong acids, support the desired current flow.
  • the electrolyte is typically a composition comprising at least a solvent, e.g., water, an acid, or mixture thereof, and one or more ionic compounds dissolved therein the increase the conductivity of the electrolyte.
  • a solvent e.g., water, an acid, or mixture thereof
  • ionic compounds dissolved therein the increase the conductivity of the electrolyte.
  • non- acid, ionizable compounds that can in various preferred embodiments of this invention be added to the electrolyte composition.
  • these non-acid, ionizable compounds are added to aqueous acid or aqueous/acetonitrile formulations.
  • the invention encompasses electrolytes made with sufficient, i.e., at least 3%, preferably at least 10%, e.g., at least 20%, of an acid such as phosphoric acid, nitric acid, sulfuric acid, strong organic acids, e.g., acetic acid, hydrochloric acid, or mixture thereof.
  • an acid such as phosphoric acid, nitric acid, sulfuric acid, strong organic acids, e.g., acetic acid, hydrochloric acid, or mixture thereof.
  • Wlrier electrolytes useful in standard elecfropolishing (EP) can be used, some specialized electrolytes provide additional advantages.
  • one or more reduction reactions may occur at the cathode.
  • aqueous acids these may include electrolysis of water to liberate hydrogen and the reduction or plating of metal ions derived from the anode.
  • metal salts e.g., CuSO
  • metal salts can be included in the electrolyte, hi conventional EP, where the work piece is totally immersed in elecfrolyte
  • metal salts generally are not added because the cost of formulating, recovering and/or disposing of such large volumes of material, would be prohibitive. Additionally, the high concentration of metal salts would introduce contamination to the work piece.
  • a single MMEP cathode half-cell contains only a few ml of electrolyte and can be used to polish work pieces many times larger in volume.
  • One side reaction intrinsic to electropolishing is the electrolysis of water, which can produce molecular hydrogen, molecular oxygen, or both.
  • n e.g., n greater than 3, usually greater than 4, so that a sufficient amount of hydronium ions are formed to solvate the electropolished metal ions and prevent deposition of metal oxides, hydroxides, and/or carbonates.
  • a unique additional invention is an apparatus adapted to electropolish or electro-mechanically polish Cu damascene wafers wherein the solution contacting the cathode contains a sufficient concentration of reducible-metal salts to substantially suppress the formation of molecular hydrogen at the cathode, but where the electrolyte or fluid contacting the work piece has substantially less reducible-metal salts.
  • reducible-metal salts we mean any salt of a reducible transition metal that has sufficient solubility in the electrolyte and that can be reduced to metallic form a the cathode.
  • the metal salts in the electrolyte near the can comprise or consist essentially of the metal being polished.
  • divalent metals are preferred over monovalent metal salts.
  • Preferred reducible-metal ions are Cu(I), Cu(II), Ni(II), Ag(I), Fe(II), Cr(III), and/or complexes of these ions.
  • the most preferred reducible- metal ions are Cu(I) and Cu(II).
  • a preferred reducible metal salt is a copper salt, where the anion is the same as the anion of the acid component of the electrolyte, e.g., copper sulfate.
  • substantially reduce the formation of hydrogen gas we mean that at least one metal ion is reduced for every hydrogen molecule generated by the electrolysis of water at or near the cathode.
  • the amount of reducible metal ions in the "reducible-metal-containing" elecfrolyte contacting the cathode is sufficient so that 2 or more metal ions are reduced, more preferably 4 or more metal ions are reduced, and most preferably 8 or more metal ions are reduced, for example 15 to 100 metal ion are reduced, for every hydrogen molecule generated by the electrolysis of water at or near the cathode.
  • substantially less reducible metal salts we mean the molar concentration of reducible metal in fluid contacting the work piece is less than about 50% of the molar concentration of reducible metals in the elecfrolyte fluid contacting the cathode, and preferably is less than 10%, more preferably is less than about 1%, for example is less than about 0.1%, of the molar concentration of reducible metals in the electrolyte fluid contacting the cathode.
  • a concentration of between 0.001 M to about 1 M of a reducible-metal salt is sufficient, and in preferred embodiments the electrolyte contacting the cathode comprises about 0.01 M to 0.5 M, for example about 0.05 M to about 0.3 M, alternately from about 0.1 M to about 0.3 M, of a reducible-metal salt.
  • the same criteria apply to any of the above-mentioned MMEP apparatus, and in MMEP the reducible-metal-containing electrolyte solution or gel is rigorously kept away from the work piece.
  • the conductive electrolyte solution or gel comprises a reducible-metal salt, wherein the amount of reducible-metal salt is sufficient to substantially reduce the formation of hydrogen gas.
  • the conductive electrolyte solution or gel comprises water, an acid, and a reducible-metal salt, and wherein the amount of reducible metal salt is sufficient to substantially reduce the formation of hydrogen gas.
  • the conductive electrolyte solution or gel comprises from about 0.001 M to about 1 M, preferably about 0.01 M to 0.5 M, for example about 0.05 M to about 0.3 M, or alternately from about 0.1 M to about 0.3 M, of reducible-metal salt.
  • the conductive electrolyte solution or gel comprises water, an acid, and from about 0.001 M to about 1 M, preferably about 0.01 M to 0.5 M, for example about 0.05 M to about 0.3 M, or alternately from about 0.1 M to about 0.3 M, of reducible-metal salt.
  • the electrolyte composition comprises one or more metal salts, wherein at least one metal salt comprises the metal that is being removed from the anode work piece.
  • concentration of these metal salts can range for example from 0.001 M to 5 M, or typically from 0.05 M to 2.5 M.
  • the salts can be selected by one of ordinary skill in the art having benefit of this disclosure.
  • metal salts we mean the commonly known dissociable (soluble) metal salts as well as metal salts of acidic polymers, e.g., polycarboxyhc compounds such as polyacrylates and polysulfonates.
  • metal salts we mean the commonly known dissociable (soluble) metal salts as well as metal salts of acidic polymers, e.g., polycarboxyhc compounds such as polyacrylates and polysulfonates.
  • higher metal salt concentrations in the electrolyte reduce electrolysis of water and make the system more efficient. The following are exemplary, and most will benefit from additional salts.
  • an aqueous nitric acid is preferred, having for example 0.1 M to 4 M nitric acid in water, e.g., 1 M to 2 M nitric acid in water.
  • Other electrolyte ions such as sodium nifrate and or silver nitrate may be added in an amount between 0.001 M to 1 M, or typically from 0.05 M to 0.3 M.
  • an aqueous sulfuric acid is preferred, having for example 5% to 50% sulfuric acid in water, e.g., 10% to 30% sulfuric acid in water.
  • electrolyte ions such as sodium sulfate, copper sulfate, and/or nickel chloride or the like may be added in an amount between 0.001 M to 1 M, or typically from 0.05 M to 0.3 M.
  • an aqueous sulfuric acid/acetonitrile composition is preferred, having for example 3% to 30% sulfuric acid/10% to 50% water/40% to 75% acetonitrile is useful.
  • Other electrolyte ions such as sodium sulfate, copper sulfate, and/or cobalt chloride or the like may be added in an amount between 0.001 M to 1 M, or typically from 0.05 M to 0.3 M.
  • an aqueous sulfuric acid/hydrochloric composition is preferred, having for example 0.5 M to 5M HCl/10% to 40% sulfuric acid/40% to 80% water is useful.
  • Other electrolyte ions such as sodium sulfate, copper sulfate, and/or tin chloride or the like may be added in an amount between 0.001 M to 1 M, or typically from 0.05 M to 0.3 M.
  • small amounts of HF may be beneficial.
  • polishing tungsten chelators may be particularly beneficial.
  • an aqueous sulfuric acid composition is useful, having for example 5% to 50% sulfuric acid/50% to 90% water is useful.
  • electrolyte ions such as sodium sulfate, copper sulfate, and/or ferrous sulfate or the like may be added in an amount between 0.001 M to 1 M, or typically from 0.05 M to 0.3 M.
  • an aqueous sulfuric acid/hydrochloric composition is preferred, having for example 1 M to 6M HCl/5% to 30% sulfuric acid/60% to 90% water is useful.
  • Other electrolyte ions such as sodium sulfate, copper sulfate, or the like may be added in an amount between 0.001 M to 1 M, or typically from 0.05 M to 0.3 M.
  • the MMEP electrolyte comprises metal ions equivalent to those generated at the anode, and at a concentration sufficient to allow metal ions to be plated onto the cathode at a rate equal to the rate of metal removal from the work piece, such that a steady state is maintained and the salt content of the electrolyte remains constant.
  • solvent usually water
  • solvent typically is transferred across the membrane by osmosis, electro-osmosis, and/or pervaporation, causing the salt concentration of the electrolyte to vary with time unless this solvent is removed. Adjustment for such variations may be readily made by adding or removing the appropriate amount of pure solvent.
  • the elecfrolyte can comprise one or more metal salts, wherein the metal salts comprise metal other than those metals that are being removed from the anode.
  • the concentration of metal salts that comprise metal other than those metals that are being removed from the anode work piece can range for example from 0.001 M to 1 M, or typically from 0.05 M to 0.3 M.
  • the counter-ion of the metal salt is identical to the counter- ion in the acid.
  • Counter-ions can be sulfates, phosphates, halides, e.g., chlorides, an organic acid salt such as acetate, salicylate, benzoates, citrates, and the like, provided the metal salt produces charged ions capable of facilitating current flow.
  • the salts do not react with and thereby cause the metal ions entering the electrolyte solution through the membrane to precipitate.
  • the salts can be selected by one of ordinary skill in the art having benefit of this disclosure.
  • metal ions from metal salts comprises a charge of at least positive two, for example an ion having a charge of positive three or positive four.
  • Exemplary ions include aluminum and iron ions.
  • the highly charged ion will allow current flow with less ions.
  • the metal ions must be stable in the electrolyte, and must be reducible at the cathode.
  • at least a portion of the metal ions from the metal salts are not reducible at the cathode. The advantage is that while these salts provide conductivity, they do not plate on and contaminate a cathode.
  • Exemplary salts include halides of alkali metals, e.g., sodium or potassium ions, which will not tend to be reduced at a voltage where for example a copper ion is reduced.
  • alkali metals e.g., sodium or potassium ions
  • the disadvantage is that if an insufficient concentration of reducable metal ions are at the cathode, there may be electrolysis of the solvent, e.g., the water.
  • the recovered cathode may be used to fabricate sputtering targets or plating anodes for the production of copper damascene wafers.
  • the cathode may therefore be made of highly pure copper metal.
  • the electrolyte in the cathode half-cell may comprise a conventional or commercial electroplating solution containing acids and metal salts and including various additives such as brighteners, levelers and surfactants known in the elecfroplating art to improve the integrity, smoothness and/or uniformity of electrodeposition on the cathode.
  • the metal ions in the electrolyte are partially, substantially, or fully complexed with one or more chelators.
  • Chelators include those chelators normally found in the art, e.g., EDTA, DPTA, citric acid, di- functional organic acids including for example di- or multi- hydroxybenzene-type compounds, e.g., such as catechol, butylated hydroxytoluene ("BHT"), and the like; dicarboxylic acids such as oxalic acid, malonic acid, succinic acid, glutamic acid, adipic acid, maleic acid, fumaric acid, phthalic acid, isophthalic acid, terephthalic acid and the like; tricarboxylic acids such as citric acid, aconitic acid, trimellitic acid and the like; hydoxycarboxylic acids such as glycolic acid, gallic acid, lactic acid, 2-hydroxybuteric acid, tartaric acid, malic acid, salicylic acid, and the like; ketocarboxylic acids, such as acetoacetic acid, ketoglutaric acid and the like; amm
  • the complexed metal in an electrolyte composition having chelators present, it is preferred that if the complexed metal is one that is beneficial to be reduced at the cathode, then the chelator-metal complex must be able to release the metal ion at the cathode, or otherwise permit the metal ion to be reduced. However, if the complexed metal ion is of a type where reduction at the cathode is not desired, then a stronger chelator can be used, and the complexing agent can inhibit the ability of the cathode to reduce those metal ions.
  • a source of strong base or cyanide ion can be used in the electrolyte or in the low-conductivity solution in an anodic generation of complex metal anions such as Al(OH) 6 "3 or Fe(CN) 6 "3 .
  • bases are discouraged in an aqueous environment, as the metal ions passing through the membrane may form undesirable precipitates at the membrane.
  • cyanide can create handing problems, even within the small confines of a cathode half cell.
  • other preferred complexing compounds are acetonitrile, and less preferably propionitrile, or a mixture thereof.
  • the electrolyte comprises one or more larger yet releasing chelators, including for example salicylic acid, benzoates, naphthanoates, and the like.
  • a large metal-complex will not readily diffuse through a membrane, and the membrane can act by means of ion exclusion, size exclusion, or a combination thereof.
  • chelating materials in particular crown ethers and nitrogen analogues thereof, including hydrophilic derivatives thereof, can be used in combination with certain electrolyte solvents.
  • the chelator that forms copper phthalocyanine or a hydrophilic derivative thereof such as copper phthalocyanine tefrasulfonate
  • the chelator that forms cupromeronic blue can be used in the electrolyte.
  • Various crown ethers are known to one of ordinary skill in the art, e.g., a 12-crown-4 is specific for lithium ions.
  • crown ethers and analogues and derivatives can hold certain metal ions tenaciously and not allow reduction thereof at the cathode, so embodiments that use such crown ethers to chelate metal electropolished from the surface may have the electrolyte be circulated in the half-cell so that the complexed metal ions can be extracted from the electrolyte outside the cathode half-cell.
  • the elecfrolyte solvent is not water, some salts may have special counterions to enhance solubility. For example, if pure acetonitrile is the solvent electrolyte, tertramethylammonium hexafluorophosphate is soluble enough to provide at least minimally sufficient conductivity.
  • the polymeric backbone of any metal-neutralized polymer can be modified from for example polyacrylate to instead incorporate hydrophobic or aromatic moieties, using monomers known in the art, to achieve sufficient solubility of the metal-neutralized polymers in the non-aqueous solvent.
  • Continuous Treatment Of Elecfrolyte Composition The half cell can be a closed system, but generally problems such as contamination, gas generation, heat generation, declining ratio of metal to acid, and/or dilution can be dealt with by circulating the electrolyte composition in and out of the half cell and treating the electrolyte. Such circulation can be accomplished by for example a closed pump. While the electrolyte material is outside of the half cell, it can be treated as necessary.
  • Electropolishing generates heat, so the electrolyte may be cooled as necessary to maintain the desired temperature.
  • the reaction at the anode may result in gas generation, for example from the electrolysis of water.
  • the system can include a separator or scrubber to remove gas.
  • the electrolyte may carry entrained particulates, e.g., precipitates or reduced metal. Filtering is generally sufficient to remove these particulates.
  • the polishing process may consume one or more components of electrolyte composition, and these components can be added to rejuvenate the electrolyte composition. As previously noted, during polishing solvent may be lost from or gained by the electrolyte due to transport across the membrane. Appropriate adjustments to elecfrolyte concentration may be made by addition or removal of solvent from the pumping reservoir.
  • Elecfrolyte Compositions can comprise two or more elecfrolyte solutions within the half-cell, which for example are separated by a membrane.
  • Such a system may be desirable if, for example, a first electrolyte composition is desirable at the cathode, and a second elecfrolyte composition is desirable at the interface of the membrane contacting the work piece.
  • the electrolyte contacting the membrane adapted to contact the work piece may be designed to contain very little of dissolved or solvated metal that is being electropolished from the work piece, for example to further limit potential contamination in the event of a membrane failure.
  • the electrolyte solution contacting the cathode may have a considerable quantity of dissolved or solvated metal that is being electropolished from the work piece, to allow reduction of that metal, forming a steady state.
  • two different electrolytes may be formulated from two mutually immiscible solvents, such as, for example dichloromethane (with other components to increase conductivity and metal solubility) and water, i that case, the less dense elecfrolyte may contact the one member of the half cell (membrane or cathode) while the more dense electrolyte contacts the lower member of the half cell (cathode or membrane, respectively).
  • two mutually immiscible solvents such as, for example dichloromethane (with other components to increase conductivity and metal solubility) and water
  • the less dense elecfrolyte may contact the one member of the half cell (membrane or cathode) while the more dense electrolyte contacts the lower member of the half cell (cathode or membrane, respectively).
  • a partially protective, substantially permeable structure is disposed between the charge- selective ion-conducting membrane and the work piece, wherein the protective coating has holes of sufficient size to allow sensible contact between the membrane and the work piece surface.
  • Such an optional structure may be extremely thin, e.g., on the order of about 0.01 microns to about 2 microns thick, for example between about 0.1 and 0.5 microns thick, such that the holes through which the membrane protrudes need not be large. This optional structure may contact the substrate, but it does not need to do so. Thicker optional structures can be used, but the size of the holes tlirough which the membrane must protrude must be increased accordingly.
  • the optional structure most beneficially comprises a large plurality of openings there-through so that the membrane can substantially contact.
  • the optional structure generally extends over the entire lateral extent of the contact area between the charge-selective ion-conducting membrane and the work piece.
  • the optional structure can be separate from or laminated to the charge- selective ion-conducting membrane.
  • the optional structure can be attached to or separate from the cathode half cell.
  • the optional structure can have the appearance of a window frame, honeycomb, or the like, providing support between the charge-selective ion-conducting membrane and the work piece, but not completely covering the charge- selective ion-conducting membrane and the work piece, h one such embodiment, the protective covering can have the appearance of a thin mesh.
  • the structures may be solid or hollow.
  • the optional structure is formed from plastic and may incorporate a halocarbon polymer, e.g., a fluorocarbon polymer, providing chemical inertness and a slidable surface. Any optional structure that is substantially inert and tough will suffice.
  • the optional structure can be designed to have several functions.
  • the optional structure can facilitate distribution of low-conductivity fluid between the work piece and the charge-selective ion-conducting membrane contacting the work piece, either via providing channels for fluid to flow, by providing a reservoir of low-conductivity fluid at all points, or some combination thereof.
  • the membrane generally exhibits a curved shape, for example like a blister, in part resulting from internal pressure.
  • a protective structure for example a non-woven porous sheet 0.4 microns thick that has the appearance of a very loosely woven screen or cloth, will result in parts of the membrane being held substantially 0.4 microns from the surface of the work piece, and will provide channels or pathways for low-conductivity fluid to be introduced between the membrane and the work piece.
  • the optional structure can provide one or a plurality of electrodes which may contact the surface of the substrate.
  • One limiting factor in MMEP is that it will become progressively more difficult to provide sufficient current to the thin layer of metal, e.g., copper, on the surface of the substrate being polished. If the electrical connection is only along an edge, the current will result in increasing voltage drop as the thickness of the conductor continues to decline. This can result in uneven polishing.
  • Such contacts must be non- scratching, and may be formed of noble metals, e.g., gold, conductive polymers, or both.
  • the optional structure can provide either an abrasive action or increased convection, as the textured surface of the optional structure is swept across the surface of a work piece in a "squeegee-like" manner. This may be particularly useful if surface- passifying surfactants are added to the low conductivity formulation.
  • the optional structure can at least partially protect the charge-selective ion-conducting membrane from wear caused by contacting rough areas of the work piece.
  • the optional structure can provide a low-friction contact.
  • the optional structure can be designed to gather and remove oxygen, which will be generated if n is substantially greater than 2. depending on the current density and on the value of n, oxygen generation can be a rate-limiting problem.
  • larger flow channels such as would normally bring low- conductivity fluid in from the outer boundaries of the membrane-substrate contact area will also be removing gaseous oxygen, either via 2 phase flow, or by having a supply of low-conductivity fluid flowing in a direction to help sweep water away from the contact area, or both, will be sufficient to remove oxygen for a membrane with a small area of contact.
  • the apparatus of this invention can have no protective covering disposed between the charge-selective ion-conducting membrane and the work piece.
  • Every embodiment of membrane-mediated electropolishing requires the use of a membrane which separates the electrolyte solution from the low- conductivity fluid and from the work piece.
  • the membrane is a critical component of the MMEP process.
  • the preferred membranes are charge-selective ion- conducting membranes, described below. These are preferably formed of fluoropolymers, for example, which contribute to membrane inertness and to reduced friction coefficient of the membrane over the work piece. They can be any nominal thickness, for example from about 1 to about 10 mils thick, e.g., from about 2 mils to about 7 mils thick, or what is particularly preferred from a cost standpoint is having the membrane be the thickness which Nafion ® membranes are commercially sold.
  • the exterior membrane comprises a membrane that is at least partially permeable to 100 or more the metal ions elecfropolished from the work piece, but is substantially less permeable, e.g., a factor of preferably a factor of 1000 or more, to the oppositely charged ions in the elecfrolyte.
  • the metal ions constitute one of the charged species in the elecfrolyte. That is, in order to maintain the low-conductivity of the solvent or solution covering the anode, if the metal ions are positive then the membrane must be essentially impermeable to all negative ions in the elecfrolyte.
  • the membrane in order to maintain electrical circuit required for the polishing process, the membrane must, in preferred embodiments of this invention, be permeable by the' solvated or complexed metal ions produced in anodic oxidation of the work piece.
  • One class of membranes that meet this criteria are charge-selective ion-conducting membranes. In membrane mediated electropolishing, ions must pass through the membrane, hi contrast, if, as in the method taught in U.S. Patent No. 6,653,226, these metals solvated or complexed metal ions produced in anodic oxidation are reduced at the membrane, then undesired solids will scratch and damage the work piece.
  • substantially all of the solvated or complexed metal ions produced in anodic oxidation of the work piece pass through the membrane into the electrolyte.
  • This condition corresponding to a cation transference number of 1, can be achieved by assuring that no ions of opposite charge are capable of permeating the membrane.
  • Most metals to be elecfropolished require the metal to be the anode, and the resulting ions, e.g., copper, iron, nickel, and the like, will have a positive charge.
  • the membrane must be a cation-conducting membrane, as is the case in Examples 1-5.
  • anodic oxidation may also be carried out under basic conditions in the presence of negatively charged ligands such as CN " .
  • the solvated metal ion may be a negatively charged complex anion and MMEP would require an anion-conducting membrane.
  • the overall charge is the key, and charged ligands (if any) complexed with a metal can result in a negatively charged complex.
  • Charge-selective ion-conducting membranes generally consist of solid organic polymers that bear covalently bound ionic functional groups. The bound ions constitute fixed charges that are balanced by unbound, mobile counter-ions of the opposite charge. The latter may diffuse within the membrane or migrate under the influence of an electric field to carry electric current.
  • Suitable charge-selective ion-conducting membranes include film-forming ionic polymers that are stable under the conditions of the elecfropolishing process.
  • a cation- conducting membrane for example, preferably contains sulfonic acid groups bound thereto. Carboxylic acid groups are also functional. Generally, a carboxylic acid type membrane has a greater electric resistance when compared with the sulfonic acid type fluoropolymer membrane.
  • the membranes of the current invention are distinguished from cation exchange materials in that the membranes of the current invention do not capture and hold ions insufficient quantity to provide a reservoir for the polishing process, but rather allow an ion to diffuse/be driven by gradients of electric and/or chemical potential to the other side of the membrane. Generally, this distinction has real operational differences.
  • a cation exchange membrane more aggressively strips metals from a solution, but would as it absorbed metal ions gradually lose its effectiveness (thereby changing polishing efficiency with time) and will eventually need to be regenerated, after the cation exchange membrane is so depleted of active sites that metals do not absorb thereon but rather stay in solution or precipitate and contaminate the work piece.
  • a preferred class of membranes for elecfropolishing copper are cation-conducting membranes, especially those formed from polymeric ionomers functionalized with strong acid groups, with a pKa of less than 3. Sulfonic acid groups are preferred strong acid groups.
  • Preferred polymeric ionomers are copolymers of fluorinated and/or perfluorinated olefins and monomers containing strong acid groups.
  • the membrane is layered, and comprised fluoropolymer membrane comprising at least two integrally laminated layers including a first layer made of a perfluorocarbon polymer having carboxylic acid groups as its ion exchange groups, and a perfluorocarbon polymer having sulfonic acid groups as its ion exchange groups.
  • the layers can be separated with fluid therebetween.
  • the membrane can be a single layer having both sulfonic and carboxylic groups, e.g., those made by the copolymerization of a carboxylic acid type monomer with a sulfonic acid type monomer, or by the copolymerization of a carboxylic acid type monomer with a sulfonic acid type monomer, or by impregnating a sulfonic acid type fluoropolymer membrane with a carboxylic acid type monomer, followed by polymerization.
  • Another membrane can be formed from a blend comprising a sulfonic acid group-containing polymer and a carboxylic acid group-containing polymer, is laminated on a sulfonic acid group membrane, as described in U.S.
  • Perfluorocarboxylate ionomer membranes or Nafion ® perfluorosulfonate ionomer membranes are composed of fluorocarbon chains bearing highly acidic carboxylic and sulfonic acid groups, respectively. On exposure to water, the acid groups ionize, leaving fixed sulfonate anions and mobile hydrated protons. The protons may be readily exchanged with various metal cations.
  • Nafion ® is particularly well suited for use in MMEP due to its strong common-ion exclusion, high conductivity, strong acidity, chemical stability and robust mechanical properties.
  • MMEP polishing of Cu in water requires an acidic environment at the surface of the anode because at pH greater than 4, Cu +2 precipitates from water as CuO or copper hydroxide. Nafion provides the necessary acidic environment to solubilize Cu +2 .
  • An anion-conducting membrane for example, preferably contains for example quaternary ammonium groups bound thereto. Tertiary or lower amino groups are also functional.
  • Strongly basic styrenic ion-conductive membranes can be formed from for example crosslinked copolymers of styrene and divinylbenzene, to thereby synthesize a crosslinked poly-styrene-divinylbenzene, then chloromethylating this crosslinked polymer using a Lewis acid and causing a tertiary amine or the like to add thereto.
  • Such strongly basic resins can be used in ion-conductive membranes in the whole pH range and therefore are utilized in a wide range of applications.
  • Methods for making anion- conducting membranes can be adapted from methods to make anion-conductive membranes with little experimentation. See, e.g., U.S. Patent No. 6,646,083.
  • Such membranes are commercially available. To the extent that such charge-selective ion-conducting membranes transport only ions of one sign, this provides two unique advantages for MMEP over EP. Firstly, it prevents contaminating the work piece and low-conductivity solvent by electrolyte. Secondly, since the only source of mobile ions on the anode side of the membrane is the anodic oxidation reaction, all of the current will be carried by these ions. Consequently, these ions must be absorbed and transported through the membrane and removed from work piece at exactly the same rate as they are produced, thereby eliminating a second source of contamination.
  • the process can include both anion and cation conducting functionalities in a membrane, as described for example in International Publication No. WO 03/098676.
  • a dual function membrane is totally inoperative, as it would allow conductive ions to pass from the elecfrolyte, through the membrane, and into the low conductivity solution, h addition, the exceptional planarization efficiency of the MMEP process, which distinguishes this invention from other forms of electropolishing, apparently derives from the close relationship between the transport of solvent and anodically generated metal ions across the membrane. This valuable feature of the MMEP process maybe severely compromised or lost if a significant fraction of the current is carried by other ions.
  • the low-conductivity fluid must generally wet the work piece.
  • hydration or analogous solvation
  • metal ions migrate away from the interface they carry with them solvent molecules, thus creating a partially dehydrated (or de-solvated) boundary layer within the membrane.
  • MMEP was first attempted using a Cu work piece immersed in water under static contact with a Nafion ® membrane (see Example 1). Under these conditions it was discovered that the membrane developed a strong adhesion to the work piece which persisted for as long as the current was allowed to flow and that the polishing process was largely restricted to the periphery of the contact area. Without being bound by theory, both of these effects are understood to be a consequence of electro-osmotic membrane dehydration. Since each Cu +2 ion is solvated by at least four water molecules, transport of Cu +2 ions also removes water from the membrane.
  • the polishing face contains low-conductivity fluid-permeable members or areas interspersed among the polishing membranes, wherein these low- conductivity fluid-permeable members have a pressurized source of low-conductivity fluid, e.g., water.
  • the membrane can be similar to the screen or gauze-like protective coverings. Channels extending through the half cell can provide low-conductivity fluid to the interface between the membrane and the work piece, and additionally can provide a method of controlling temperature gain within the half cell.
  • the exterior membrane can separate the electrolyte from the low-conductivity fluid by some mechanism other than charge-exclusion.
  • Two other mechanisms known to be useful are size exclusion and polar/nonpolar differentiation.
  • size exclusion generally, ions in the electrolyte composition must be large, while ions generated from the work piece and passing into the low-conductivity fluid must be small.
  • One method of making ions large is, for example, to chelate or complex the ions with large chelator compounds.
  • metal-neutralized hydrophilic polymers such as polymers having carboxylate, sulphonate, and/or other acidic moieties attached to tha polymeric chain
  • size exclusion embodiments Any common micro-porous membranes, such as those sold by Millipore, Pall, and the like, will be operable.
  • the size exclusion membrane may be treated to increase the acidity of the pores within the membrane, for example by impregnating the membrane with a small amount of fluid comprising an acidic low molecular weight polymeric material.
  • size exclusion is less effective than the charge-exclusion membranes, however, as size of pores can change with stress.
  • Membranes are known in the art that keep separate polar liquids from non-polar liquids. To the extent the metal ions can be solvated in the non-polar liquids, such a separation of elecfrolyte from low-conductivity fluid can be maintained based on polarity difference, e.g., the lack of miscibility of the two' fluids. Again, separation based on polarity is not preferred, however, as it is not robust, and as it is difficult to solvate metal ions in a non-polar fluid without using large chelators, e.g., crown ethers, hydrophillic polymers, and the like.
  • large chelators e.g., crown ethers, hydrophillic polymers, and the like.
  • Ion-Conducting Membranes In the preferred embodiments of the invention, there is only one membrane disposed between the cathode and the subsfrate being polished. As described infra, however,in some embodiments of this invention there are a plurality of ion-conducting membranes disposed between the cathode and the work piece. Generally, charge specific ion conducting membranes are preferred, but the internal membranes can be more specific. Generally, the external membrane, e.g., the membrane that contacts an elecfrolyte composition on one side and contacts the low- conductivity fluid/work piece on the other side, is permeable to all metals elecfropolished from the work piece.
  • Internal membranes that contact different electrolyte compositions on either side typically do not need to be as robust as exterior membranes.
  • Internal membranes can be specific to one or more metals, excluding other metals. This is useful if a plurality of metals are being elecfropolished from a substrate, but some of these metals if they reduce at the cathode will poison a cathode. These metal-specific membranes are also useful if it is not desired to reduce one or more metals in the elecfrolyte for other reasons, for example the value of the metal or environmental difficulties associated with handling the metal.
  • Internal membranes can be specific to hydronium ions, excluding all metal ions.
  • the electrolyte composition contacting the external membrane has very little of metals that are being electropolished from the work piece, e.g., the elecfrolyte composition comprises an aqueous solution of sulfuric acid but little copper sulfate, while the electrolyte composition contacting the cathode had a substantial quantity of added metal salts, e.g., an aqueous solution of sulfuric acid and copper sulfate.
  • such an embodiment will substantially reduce metal ion leakage from the cathode half cell, but will also provide sufficient reducible metal ions, e.g., copper ions, at the cathode to prevent formation of hydrogen gas.
  • the power supply is substantially a DC power supply. Electropolishing only occurs when the work piece is an anode, and therefore a DC power supply would seem to be sufficient.
  • the power supply is advantageously variable and reversible. It is therefore advantageous for the power supply to be able to supply short-lived variations or pulses in the supply voltage or current. In particular, reducing the voltage from the designed efficient voltage to a value near zero at regular but short-lived intervals increases planarization efficiency under certain polishing conditions.
  • a variation of this apparatus includes the embodiment wherein the substantially DC electric power source provides a pulsed DC current alternating between a high current value and a low current value that is less than 50% of the high current value.
  • the substantially DC electric power source may advantageously provide a pulsed DC current alternating between a high current value and a low current value that is less than 50% of the high current value, wherein the low current value is maintained for a period of time between about 10 microseconds and about 2 seconds.
  • a low current (or preferably no current) time t 0 less than 10 microseconds is probably dominated by charging current (capacitor effects), and a time greater than 2 seconds is simply wasteful as little effect is noted with increasing t 0ff . above a few seconds.
  • the high current state can be advantageously maintained at an interval between about 10 microseconds and about 5 seconds. Again, times less than 10 microseconds are dominated by charging current and t on times greater than 5 seconds start approaching steady state behavior, hi a preferred embodiment, the electrical power source advantageously provides a pulsed DC current alternating between a high current value and a low current value that is less than 50% of the high current value, wherein the low current value is maintained for a period of time between 50 microseconds and 0.1 seconds and the high current value is maintained for a period of time between about 100 microseconds and 0.2 seconds.
  • MMEP can be used to planarize large features on Cu damascene wafers to within typical industry specifications, without the need for specialized plating technology or mechanical polishing.
  • the "low current value" can even be made negative, e.g., by providing a short-lived reversal of the imposed voltage.
  • a pulse can be provided by reducing the voltage to negative 2 volts, or negative 1 volt, or zero volts, for a short period of time. Without being bound by mechanisms or theories, this may be due to the influence of concentration gradients of protons, metal ions and/or water.
  • the short-lived voltage "reduction" can also be obtained by applying a reversed voltage, e.g., instead of reducing the voltage to 2 volts, 1 volt, or zero volts, the voltage can be set to provide a short-lived pulse of voltage negative 1 volts, for example. While in theory this may result in plating of metal on the work piece, there is little or no metal in the low-conductivity fluid that can be plated, and the time duration is such that no plating should occur. Operating- with negative voltages is to be avoided generally as it can cause metal dentrites to grow in the membrane.
  • the electric power source need only be a "substantially DC electric power source,” which is a power source adaptable to provide a DC current wherein the current is integrated over a sufficient time, and where that term encompasses a DC power source, a pulsable DC power source, and a pulsable DC power source wherein pulses may be of the opposite polarity.
  • the product of times the interfacial velocity represents the length of new surface brought into contact during the recovery period between polishing periods. This "rehydration length" was found to have a systematic effect on the stoichiometric number n.
  • excess water is present at the interface it can be anodically oxidized in competition with oxidation of Cu, resulting in n>2 electrons/Cu .
  • n was found to converge to 2 as the product of t 0f f and interfacial velocity approached the width of the contact area. This indicates that the relative motion of work piece and membrane is the primary mechanism for rehydrating the interface during t 0ff under pulsed operation.
  • the source of low-conductivity fluid can be modified to take advantage of this phenomenon.
  • the supply of low-conductivity fluid to the work piece surface can be uniform, for example pumped or a gravity feed system, it may be possible to pulse the source of low-conductivity fluid so that a short-lived excess is present during the t o ff, allowing faster re-hydration of the membrane, and the supply of low-conductivity fluid can be reduced accordingly during the t on when partial dehydration of the membrane provides greater planarization efficiency.
  • a pulsed source can be obtained from any pump providing a timed pulse, and the duration of the pulsed supply of low-conductivity fluid can be matched to the t on and t 0ff of the electric power supply.
  • the movement of the membrane against the work piece surface can be made somewhat more intermittent and also be timed to t on and t 0ff of the substantially DC power supply. Such movement can be obtained for example by utilizing stepper motors to drive the movement mechanism.
  • the work piece can comprise any metals and metal alloys.
  • Suitable metals include silver, nickel, cobalt, tin, aluminum, copper, lead, tantalum, titanium, iron, chromium, vanadium, manganese, zinc, zirconium, niobium, molybdenum, ruthenium, rhodium, hafnium, tungsten, rhenium, osmium, iridium, or alloys such as brass, lead/tin alloys, steel, and alloys thereof, compounds such as oxides and/or nitrides thereof, and mixtures thereof, e.g., more than one of the aforesaid metals or metal compounds.
  • Preferred methods comprise polishing work pieces comprising silver, nickel, cobalt, tin, aluminum, copper, lead, and alloys such as brass, steel and/or lead/tin.
  • MMEP can be used to polish various different metals using strong acid elecfrolytes in a MMEP cell with a Nafion ® membrane. Differences in the coordination chemistry and solubility of different metal ions may require the use of different electrolytes for different metals.
  • a source of strong base or cyanide ion can be used in the elecfrolyte or in the low-conductivity solution in an anodic generation of complex metal anions such as Al(OH) 6 "3 or Fe(CN) 6 "3 .
  • MMEP would require an anion-conducting membrane instead of a cation- conducting membrane.
  • Alternate Polishing Methods In a mode of electropolishing different than MMEP, the solvated or complexed metal ions produced in anodic oxidation of the work piece remain in the low- conductivity fluid, and a substitute ion, e.g., hydrogen (H ), passes through the membrane into the elecfrolyte.
  • H + must be provided by for example electrolysis of water or by an acid in the low- conductivity fluid.
  • the conductive ions in the electrolyte solution comprise a reducible metal, as hydrogen reduction at the cathode is sfrongly discouraged.
  • This electropolishing method is distinguished by the membrane permeability to an ion (H 1" ) controlling the electropolishing rate as ion migration through the membrane is essential to the elecfroplating.
  • This method is not preferred because removing the solvated or complexed metal ions produced in anodic oxidation from the low- conductivity fluid is required. Generally, such a process will result in the metal ions that has been oxidized quickly combining with available oxygen to form metal oxides.
  • FIG. 10 shows cathode half-cells with one and two membrane "windows" for polishing 4" x 4" metal wafer samples on a spin-polishing tool. In both cases, elecfrolyte solution is continuously circulated through the half-cell cavity by means of a peristaltic pump.
  • a continuous flow of low-conductivity fluid e.g., de-ionized water
  • a continuous flow of low-conductivity fluid is delivered into the interface where the membrane is moving over the surface, such that some water is trapped between the advancing membrane and the substrate surface, i.e., in a very thin gap between the cathode half-cell and the work piece.
  • the cathode half-cell contains a cathode, typically a Cu cathode, in contact with electrolyte solution, e.g., an aqueous composition containing acid and a copper salt , for example 0.8 M CuSO 4 , or Cu(CN) 3 "2 , or mixture thereof, and 0.5 M H SO 4 in water.
  • electrolyte solution e.g., an aqueous composition containing acid and a copper salt , for example 0.8 M CuSO 4 , or Cu(CN) 3 "2 , or mixture thereof, and 0.5 M H SO 4 in water.
  • the Faradaic current "i" is carried exclusively by cations passing from the work piece into the electrolyte.
  • concentration of ions in the low-conductivity solution e.g., in the de-ionized water, is far too low to support the current required for elecfropolishing when there is any appreciably thick intervening layer of low-conductivity fluid.
  • Equation 1 shows the oxidation of copper, which in the ionic form is complexed with about 6 water molecules.
  • Equation 2 shows the anodic oxidation of water to produce Oxygen gas and two protons in the form of solvated hydronium ions, such as H 9 O 4 species, which also contains 4 water.
  • concentration of Cu +1 is considered to be negligible because in the absence of stabilizing ligands, it is thermodynamically unstable with respect to disproportionation.
  • Reaction 1 is the desired elecfropolishing process
  • reaction 2 is a desired electrolysis of water. It is possible to oxidize Cu at potentials low enough to avoid reaction 2. This will provide the greatest coulombic efficiency, i.e., the most copper oxidized and removed per unit of current flow. But planarization of Cu at high removal rates by the Membrane- Mediated Electropolishing process requires sufficiently high voltages that both reactions occur in varying degree.
  • Coordinated water molecules migrate with the cations from the surface of the anode across the membrane and into the cathode half-cell by the process of electro-osmosis.
  • the above equation also provides an indication of the rate of dilution of the elecfrolyte composition within the cathode half-cell. It should be recognized that even in the absence of electrical current water may be carried into the cathode half-cell by simple osmosis, or may be forced out of the half-cell by the hydrostatic pressure (reverse osmosis). Mechanical transport of water to the interface area between membrane and substrate by movement of the work piece with respect to the membrane appears to be the most important re-hydration process.
  • electro- osmotic dehydration of the contact area should be balanced by diffusion of water througli the membrane and by hydrodynamic transport from the reservoir of de-ionized water surrounding the contact.
  • An important consideration is the acidity of the reaction zone.
  • the most preferred low-conductivity fluid is very pure water.
  • Cu +2 has a very low solubility in water at pH 6 to 7.
  • Aqueous Cu +2 precipitates as CuO or Cu(OH) 2 , according to the equilibrium reactions 4a and 4b.
  • the solubility of Cu in pure water is very low. Precipitation of CuO or Cu(OH) 2 is undesirable because solid particles may contaminate or abrade the surface of the work piece, or foul the membrane.
  • the low solubility of copper ions limits mass transport and current density.
  • the very low solubility of CuO and Cu(OH) 2 in unbuffered water also limits the flux of Cu +2 to the cathode and therefore limits the current density which may flow through a water layer separating the surfaces of the work piece and membrane.
  • a very dilute aqueous acid solution as the low conductivity solution rather than deionized water, to help prevent precipitation of for example copper ions or other metals which may form in the presence of water at neutral or basic pH.
  • Many metals ions are stable in an acidic environment, and we explained the benefits of having the low-conductivity fluid further comprise acid sufficient to obtain a pH of between 2 and 6.9, for example about 4 to about 6.5 or about 5 to about 6.5.
  • useful acids can include for example sulfuric acid, carbonic acid, hydrochloric acid, nitric acid, phosphoric acid, pyrophosphoric acid, carboxylic acids, or mixtures thereof.
  • the low pH provided by acids is not as beneficial as generating protons by partial electrolysis of water.
  • Addition of acid to the low- conductivity fluid causes an increase in conductivity which allows current to flow outside of the contact area. This results in etching and produced a rough surface.
  • the initial pH is not a sufficient criterion for avoiding CuO precipitation. If a fixed amount of acid is added at low concentration to the low-conductivity fluid, the acid will soon be locally depleted by reactions with CuO or Cu(OH) 2 and/or be transported through the membrane into the electrolyte. The use of acids is therefore not usually desirable.
  • An un-buffered dilute acid at low pH 1) can only dissolve a minute amount of CuO or Cu(OH) after which the pH will increase and more CuO or Cu(OH) will precipitate, and 2) results in increased conductivity and a rougher finished surface, and 3) will tend to increase the amount of metal ions that remain in the low-conductivity fluid rather than be transported through the membrane.
  • the solubility of most metal ions, and copper ions in particular, increases substantially with decreasing pH.
  • the pH is determinative of the metal-ion solvating capacity of an aqueous low-conductivity fluid.
  • the pH required to avoid precipitation is dependent on the copper ion concentration, and at very high polishing rates (high removal rate) claimed here, it is necessary to maintain a pH of less than about 5, for example about 4 or below to avoid CuO or Cu(OH) 2 precipitation.
  • the sulfonic acid groups on the Nafion ® membrane P f -SO 3 H
  • the sulfonic acid groups on the Nafion ® membrane P f -SO 3 H
  • the pH will increase well above the solubility limit.
  • Tins situation may occur, for example, when the rate of Cu +2 generation exceeds the rate of proton diffusion through the membrane from acid elecfrolyte on the opposite surface.
  • CuO may passivate the surface of the work piece, thereby limiting the rate of oxidation.
  • CuO may form on or in the membrane, thereby impairing the efficiency of the membrane in transporting the polished metal ions.
  • CuO or Cu(OH) are also a potential sources of particulate contamination and defects. It is therefore a high priority to avoid, or to severely limit the extent of CuO or Cu(OH) 2 precipitation. However, the situation changes dramatically when protons are generated at the work piece by anodic oxidation of water.
  • Equation 4 suggests one mole of CuO or Cu(OH) 2 may be dissolved for each two protons generated by the oxidation of water.
  • n is equal to or greater than 4 e-/Cu +2
  • the rate of proton generation is sufficient in most circumstances to completely suppress the precipitation of CuO, Cu(OH) 2 , and/or CuCO 3 , to increase the concenfration of Cu +2 in any interfacial water layer by maintaining a low pH, and/or replenish the acidic exchange sites in the membrane, thereby assisting in metal ion transport through the membrane.
  • the protons formed by electro-oxidation of water have no appreciable counter-ions, and therefore do not increase the conductivity of the low- conductivity fluid as much as would an equivalent amount of acid mixed with the low- conductivity fluid. Also in the absence of counter-ions the protons pass through the membrane, maintaining the acid balance in the membrane so the copper ions formed from oxidation of the substrate do no precipitate within the membrane.
  • Figure 16-A shows the dependence of RR and n on current density I, where the current density varied from about 600 mA/cm 2 to about 2800 mA/cm 2 ;
  • Figure 16-B shows the dependence of I and n on voltage for the single-window half-cell experiment of Figure 16-A;
  • Figure 16-C shows the variation of removal rate and current density versus voltage.
  • Figure 17 shows the data of Figure 16-A, and also shows the dependence of RR and n on current density I for a dual window polisher described in Example 21.
  • Figure 16-B shows that, at voltages between about 13 V and about 19 V, that the n value ranges between about 4 to about 6.
  • n between about 2 and 3 were obtained in earlier experiments at voltages below 7V and I below 500 mA/cm 2 .
  • the data from Figures 16-A and 16-B suggest that for this system n will be greater than 3.9 if either the current density is greater than about 1000 mA/cm 2 , or if the voltage is greater than about 15 volts. Further, the data from Figures 16-A and 16-B suggest that for this system n will be greater than 5 if either the current density is greater than about 1500 mA/cm 2 , or if the voltage is greater than about 16 volts.
  • n is greater than 3.9, more preferably about 4.0 or greater, for example between about 4 and about 9.0 or alternately about 4.5 and about 8.0. Similar n values are preferred if the low-conductivity fluid comprises water and one or more ligands capable of stabilizing for example copper(I) ions. If a portion, e.g., at least a tenth, for example, of the copper polished from the surface of the substrate is stabilized as copper( ⁇ ) ions, then preferred n values will be between that required for the above two scenarios.
  • the process described herein is limited to polishing copper, e.g., metal comprising more than about 50% by weight Cu, preferably more than 80% by weight Cu, and typically more than 95% by weight Cu.
  • Other metals may also be on the substrate, and some of these metals may also be polished using the methods described here.
  • the invention and parameters described here are for conciseness limited to polishing the copper portion of the subsfrate surface. It shall further be understood that many metal ions other than copper also exhibit increased solubility at low pH, so that the anodic oxidation of water would have similarly beneficial effects for membrane- mediated electropolishing of such metals.
  • the low-conductivity fluid comprises water and one or more acids in an amount sufficient to provide a low-conductivity fluid pH between about 2 and 6.9, preferably between about 4 and about 6.9, or between about 6.5 and about 6.9. More preferably, the low-conductivity fluid is water substantially free of acid, wherein protons are created via the electrolysis of water to provide a local pH between about 2 and 6.9, preferably between about 3.5 and about 6.5, or between about 4 and about 5. With the electrolytic formation of protons in the low-conductivity fluid, the ability of the low- conductivity fluid to dissolve and transport copper ions is increased, and the thickness of the layer of low-conductivity fluid between the membrane and the substrate surface is beneficially reduced.
  • anodic oxidation of water in parallel with the oxidation of Cu as reflected by n>3.5, preferably if n>4 e/Cu, should provide any or all of the following beneficial effects for the MMEP process with a Cu work piece: 1) suppress precipitation of CuO, Cu(OH) 2 , and CuCO 3 ; 2) maintain acidic exchange sites in the membrane; 3) increase current density and removal rate at any given voltage, and 4) minimize the thickness of interfacial water layers.
  • n is greater than 3, preferably greater than about 3.5, more preferably greater than about 4, for example equal to or greater than about 4.5.
  • n it is not beneficial to increase the value of n without limit.
  • the evolution of Oxygen bubbles at the surface of the substrate may be sufficiently great as to disrupt the interface causing non-uniform polishing.
  • the resulting increase in I may ultimately give rise to heating and other undesirable effects.
  • the removal rate RR is proportional to the ratio (I/n). Accordingly any change in conditions (such an increasing V) which increases n, will only increase RR if it also produces a proportionally greater increase in I.
  • the optimum value of n will therefore be dictated by both fundamental and practical limitation. In practice, we have found that MMEP on Cu can be usefully operated at high RR with values of n between 3.5 and 9.
  • n is greater than 4, say for example between 4.1 and 8.
  • the preferred n includes the number of electrons needed to oxidize the metal to the thermodynamically stable ion, and also the amount of protons needed to solubilize the metal ions in the low-conductivity fluid. For copper, this value is about 4. Higher n values may be more preferred, however, because the higher resultant flux of protons through the membrane may facilitate transport of the metal ions through the membrane.
  • Membrane mediated electropolishing of metal, for example copper, from a subsfrate surface including the steps of: a) providing a subsfrate comprising a surface having metallic copper disposed thereon; b) providing a low-conductivity fluid consisting essentially of de-ionized water to the area on the surface to be polished, so that the surface includes a layer of low- conductivity fluid disposed thereon; c) movably contacting the subsfrate surface having the low-conductivity fluid disposed thereon with a cathode half-cell which includes: i.) a cathode electrically connected to the negative terminal of an electrical power supply; ii.) a conductive electrolyte composition maintained at a hydrostatic pressure Ph, wherein said cathode is in contact with the conductive elecfrolyte composition; and iii.) a cation-selective, ion-conducting membrane having two sides and situated so as to contact on a first side the conductive
  • the current density I defined as the current divided by the contact area between the membrane and the substrate (i/A c ), is advantageously greater than about 500 mA cm 2 , for example equal to or greater than 800 mA/cm 2 , and the stoichiometric number n is between about 3.9 and about 10 electrons per atom of copper removed.
  • the ratio A c /A p is advantageously greater than or equal to about 0.02.
  • Another aspect of the invention is a method of membrane-mediated electropolishing of a substrate surface having therein a metal containing copper, said method including: a) providing the substrate having a surface comprising a metal, wherein the metal consists essentially of copper, wherein at least a portion of the metal on said surface is electrically connected to a positive terminal of a substantially DC electrical power supply, and wherein the surface is at least partially wetted with a low-conductivity fluid comprising water and having a conductivity of less than about lxl 0 "3 S/cm; b) providing a cathode half-cell comprising a cathode electrically connected to a negative tenninal the power supply, a cation-selective membrane having interior and exterior sides, and a conductive elecfrolyte having a conductivity of greater than about lxlO "1 S/cm in electrical contact with the cathode and with the interior side of the membrane, wherein the cation-selective
  • the stoichiometric number n is advantageously between about 4 and 11, preferably between about 4.5 to about 7.5 electrons per atom of copper elecfropolished from the substrate surface.
  • the current density I is greater than about 1 A/cm 2 , for example between about 1.5 A cm 2 and 3.5 A/cm 2 .
  • the voltage V is 8 Volts or greater.
  • the ratio A c /A p is advantageously greater than about 0.02, preferably equal to or greater than 0.03, for example equal to or greater than about 0.05.
  • the slope of a polarization curve at the voltage V is at least twice the average slope of the polarization curve from 3 Volts to 7 Volts.
  • the slope of the polarization curve has a positive second derivative at V.
  • a first aspect of the invention is a method of membrane-mediated electropolishing copper from a subsfrate at a removal rate of about 700 nm/min or more.
  • the process variables are controlled so that the'Membrane- Mediated Elecfropolishing process will provide a copper RR equal to or greater than 500 nm/min, preferably equal to or greater than 700 nm/min, for example equal to or greater than 800 mn/min.
  • the process variables are controlled so that the Membrane-Mediated Elecfropolishing process will provide a copper RR equal to or greater than 400 nm min, wherein n is greater than 3.9, for example from about 4 to about 11.
  • the process variables are controlled so that the Membrane-Mediated Elecfropolishing process will be conducted at a voltage V and an interfacial velocity v wherein the current density I increases with an increase in the interfacial velocity v.
  • V voltage
  • v interfacial velocity
  • Figures 5 shows a plot of I versus V at 0 velocity and at four different interfacial velocities ranging from 19 cm/s to 56 cm/sec.
  • V cr the midpoint of the range where, at voltages of 3 or more volts below the lowest voltage in the range, an increase in the interfacial velocity v from 20 cm/sec to 40 cm/sec results in no change or in a decrease in the current density I, and where, at voltages of 3 or more volts above the highest voltage in the range, an increase in the interfacial velocity v from 20 cm/sec to 40 cm/sec results in an increase in the current density I.
  • V cr In the tests above and below the voltage range defining V cr that the parameters other than velocity and voltage, including contact pressure, area of the membrane being exposed to the subsfrate surface, and even the period, magnitude, and duration of voltage pulses if pulsing will be used during commercial use. With normal polishing apparatus, one can determine this velocity V cr with a series of tests requiring a minimum of experimentation.
  • V cr ' the velocities selected for the determination are about 5 cm/sec, preferably about 10 cm/sec, above and below the expected interfacial velocity that will be used for commercial applications, instead of the arbitrarily selected 20 cm sec and 40 cm/sec used above.
  • the value V cr ' is then used in the same manner as V cr .
  • one embodiment of this aspect of the invention is the membrane- mediated electropolishing of copper from a substrate wherein the voltage is near or above V cr , for example from about 4 volts below to about 20 volts above V cr .
  • copper is polished from the substrate surface using voltage(s) in the range between about 3 volts below V cr to about 5 volts above V cr , for example in the range about 2 volts below V cr to about 3 volts above V cr . hi this voltage range, there is very little effect of minor variations in the interfacial velocity v on the current density I.
  • copper is polished from the subsfrate surface using voltage(s) about equal to or greater than V cr , for example from voltages of about 2 volts below V c ⁇ , to about 25 volts above V cr , alternately between about V cr to about 15 volts above V cr . hi this voltage range, the removal rate of the copper from the substrate is maximized.
  • copper is polished at a voltage such that the slope of the polarization curve is greater than about 50 mA/cm 2 per volt, preferably greater than about 75 mA/cm 2 per volt, alternately greater than 50 mA/cm 2 per volt, for example between about 75 ma/cm 2 per volt and about 300 mA/cm 2 per volt.
  • the polarization curves in the examples show that for most velocities, the slope of the polarization curve increases with increasing I, at least until a certain voltage is reached. In some tests, this change in slope abruptly reverses, and it is beneficial to polish at a voltage below the voltage where this phenomenon occurs.
  • V sq There is usually a voltage V sq where an audible squeaking type sound is produced as the membrane moves across the substrate surface. Generally, as the voltage is increased above V sq there will be a point where the membrane sticks to the surface.
  • copper is polished from a subsfrate surface at voltage(s) below V s , for example from voltages of about 3 volts below V cr to about 1 volts below V sq , alternately between about V cr to about 1 volts below V sq , or between about 3 volts above V CT to about 3 volts below V sq .
  • the Membrane-Mediated Elecfropolishing process when used to polish copper utilizes a voltage V of 10V or greater, preferably 14V or greater, for example from 16 to 20 V, wherein the process has the following characteristics: 1) a cathode being in electric contact with an elecfrolyte, the elecfrolyte typically having a conductivity of about 0.1 to about 2 S/cm, for example about 0.2 to about 0.8 S/cm, and a thickness (measured along a path of current flow) typically between about 1 mm and 100 mm, and/or providing a resistance typically between about 0.05 and about 9 9
  • ⁇ -cm for example about 0.5 to about 5 ⁇ -cm ; 2) a membrane being in electric contact with the elecfrolyte on a first side and with the subsfrate surface wetted with a low-conductivity fluid on a second side, the membrane typically having a conductivity less than about 5xl0 "3 S/cm in a dehydrated state, greater than about 5xl0 "3 S/cm in a hydrated state and having copper ions therein, 9 1 and about 5x10 " S/cm to about 5x10 " S/cm in a hydrated, fully protonated state, and a thickness (measured along a path of current flow) between about 20 ⁇ m and 500 ⁇ m, and/or providing a resistance typically between about 0.01 and about 50 ⁇ -cm , for example about 0.2 to about 20 ⁇ -cm 2 , such that the total resistance between the cathode and the exterior surface of the membrane is between about 0.06 to about 100 ⁇ -c
  • the Membrane-Mediated Elecfropolishing process operates at a nominal current density, defined as total current divided by the area of contact between the membrane and the work piece surface, greater than about 0.5 A/cm 2 , preferably greater than about 1 A cm 2 , more preferably greater than about 1.5 A/cm 2 , for example between 1.5 A/cm 2 and 3.5 A/cm 2 , alternatively between 1.0 A/cm 2 and 2.5 A/cm 2 .
  • a value of I between 400 mA/cm 2 and 2 A/cm 2 more typically between 400 mA/cm 2 and 2 A/cm 2 , will provide a user-specified metal removal rate.
  • the ultimate upper bound on the current density is determined by specific physical, chemical, and mechanical details of the system.
  • the metal removal rate is very high, and it may be desirable to reduce the metal removal rate as the endpoint of polishing is neared, to give more precise confrol.
  • the membrane mediated electropolishing process may be separated into two (or more) steps, with tlie first step being a mass removal at a high current density, e.g., greater than about 1 A/cm 2 , more preferably greater than about 1.5 A/cm 2 , for example between 1.5 A/cm 2 and 3.5 A/cm 2 , alternatively between 1.0 A/cm 2 and 2.5 A/cm 2 ; and one or more subsequent steps involving a lower current density, e.g., between about 0.05 and about 1.5 A/cm 2 ; for example between 0.1 and about 0.5 A cm 2 .
  • the subsfrate comprises copper, which is typically but need not be greater than about 95% by weight pure copper.
  • the substrate being polished is a copper damascene wafer.
  • n is greater than 3.9, preferably about 4.0 or greater, for example between about 4.0 and about 8.0.
  • the low-conductivity fluid comprises water and one or more ligands capable of stabilizing a portion, e.g., at least a tenth, for example, of the copper polished from the surface of the subsfrate as copper(I) ions.
  • the low-conductivity fluid comprises water and one or more acids in an amount sufficient to provide a low-conductivity fluid pH between about 2 and 6.9, preferably between about 4 and about 6.9, or between about 6.5 and about 6.9.
  • the Membrane-Mediated Electropolishing process operates at a nominal current density, defined as total current divided by the area of contact between the membrane and the work piece surface, greater than about 0.5 A/cm 2 , for example greater than about 0.8 A cm 2 , preferably greater than about 1 A/cm , for example between 1 A/cm 2 and 3 A/cm 2 , alternatively between 1 A/cm 2 and 2.5 A/cm 2 .
  • Method of Polishing Maintaining High Current Density and High Removal Rate One object of this invention is to provide the conditions wherein high polishing rates can be achieved.
  • the process variables are controlled so that the Membrane-Mediated Elecfropolishing process will provide copper RR equal to or greater than 400 nm/min, preferably equal to or greater than 600 nm/min, for example equal to or greater than 800 nm/min.
  • This polishing rate is for the entire area of the subsfrate to be polished, and this rate depends on a variety of factors, not in the least of which is ratio of the area of contact to the area polished (A c /A p ), I, and n.
  • I and n are functions of the voltage V, velocity v, and contact pressure P c .
  • one aspect of this invention is a method to membrane-mediated electro-polish a copper-containing subsfrate at a high current density and also at a high copper removal rate.
  • a principal aspect of this invention is providing a method of MMEP where the nominal current density I is sustainable at greater than 1000 mA/cm 2 , preferably greater than 1500 mA/cm 2 , for example greater than 2000 mA/cm 2 .
  • the discussion above and the examples show that the primary considerations in obtaining a high current density are having sufficient voltage and having sufficient interfacial velocity. Additionally, tool pressure, contact pressure, and membrane area each have important, although secondary, effects.
  • the process must maintain a high current density and must have a sufficiently high ratio of contacted surface area of the membrane to area polished by the process (A c /A p ).
  • RR is related to process parameters as follows: (6) where V m is the molar volume of Cu (7.135 cm 3 /mole) and F is Faraday's constant (9.65xl0 4 coulomb/mole). Equation 6 shows that RR is controlled by two ratios of process parameters.
  • the ratio (A c /A p ) is strictly geometric and subject to tool design.
  • the ratio (I/n) is intrinsically mechanistic and may vary systematically with operating parameters such as P c> v, and V.
  • Tool design provides the most direct route to optimizing RR. We anticipate that a commercial MMEP tool would incorporate multiple membrane windows in order to optimize (A A p ).
  • FIG 17 shows the results of a direct comparison of the performance of single and double- window half-cells operating in an overlapping region of I. At any given I, RR is seen to be roughly proportional to the total contact area. Most importantly, a removal rate of >800 nm/min was achieved with the double- window half-cell at a current density less than 2000 mA/cm 2 . While we have demonstrated current densities well above 3 A cm 2 , such high densities are not needed for most tool designs to realize the desired copper RR.
  • I can be sustained at rates greater than 2 A/cm 2 , it is rare that we would need to reach such a high current density to achieve the removal rates specified by integrated circuit manufacturers for the next generation of polishers.
  • a limiting value of I is achieved when the rate of electrochemical dehydration exceeds the rate of hydrodynamic water transport.
  • Figure 2-C shows a representative sequence of phases through which ions must travel to carry current from the anode to the cathode, and the ranges of ionic conductivity and thickness associated with each phase. It is assumed here that if any passivation layer is present it is too thin to introduce a significant ohmic resistance.
  • the cathode is in electric contact with an electrolyte, the elecfrolyte typically having a conductivity of about 0.1 to about 2 S/cm, for example about 0.2 to about 0.8 S/cm, and 0.25 S/cm in Figure 11-C, and a thickness (measured along a path of current flow) typically between about 1 mm and 100 mm, about 5 mm in Figure 11-C.
  • This will provide a resistance typically between about 0.1 and about 50 ⁇ -cm 2 , for example about 0.5 to about 5 ⁇ -cm 2 , and 1 ⁇ -cm 2 in Figure 11-C.
  • the membrane is in electric contact with the elecfrolyte on a first side and with the subsfrate surface wetted with a low-conductivity fluid on a second side.
  • the membrane thickness (measured along a path of current flow) typically between about 20 ⁇ m and 500 ⁇ m, alternately from about 0.2 mm to about 20 mm.
  • the membrane conductivity can vary greatly. Typically a membrane will have a conductivity of about 10 "2 S/cm to about 10 "1 S/cm in a fully protonated and hydrated state without copper ions.
  • the conductivity of hydrated and fully protonated Nl 17 Nafion ® at 25°C is 0.089 S/cm.
  • the ionic conductivity of hydrated Nafion ® fully exchanged with Cu +2 is known to be about 10 times less than the fully protonated form, e.g., about 10 "2 S/cm in a hydrated state and having copper ions therein.
  • the conductivity of Nafion ® decreases with decreasing water content, and when the last few waters of hydration are removed it becomes an extremely poor conductor.
  • a membrane will have a conductivity of about 10 "4 to 5xl0 "3 S/cm in a dehydrated state.
  • the resistance of the membrane can typically vary between about 0.01 and about 50 ⁇ -cm 2 , for example between about 0.2 to about 20 ⁇ - cm 2 .
  • low-conductivity fluid typically has a conductivity of about lxlO "7 to about lxlO "4 S/cm, for example about 10 "6 S/cm to about 10 "5 S/cm.
  • the thickness (measured along a path of current flow) varies with time and position, but the thickness is expected to be less than about 1 ⁇ m. It can be seen, however, that the anticipated range of thickness and range in conductivity will result in a variable resistance, say for example between 1 and about 100 ⁇ -cm 2 . Movement of the membrane with respect to the substrate is an essential feature of MMEP, as this is the principal mechanism by which low-conductivity fluid is supplied to the area being polished. The apparent role of interfacial velocity in re-hydrating the membrane raises the question of whether this process introduces an actual layer of liquid water between the membrane and the work piece as indicated in Scheme 1.
  • This application describes process performance in the portion of the polarization curve where I>1 A/cm 2 and dV/dl ⁇ 10 ⁇ -cm 2 , where high removal rates may be achieved, for example RR>700 nm/min, with a polishing half-cell providing Ac/A p between 0.02 and 0.1.
  • MMEP can be carried out at I greater than 1 A/cm 2 , for example greater than 1.5 A/cm 2 , and even above about 2 A/cm 2 .
  • the data from Example 4 show that high current density, e.g., >1500 mA/cm 2 , for a contact width approximately 1 cm, can be maintained if the interfacial velocity v exceeds about 20 cm/sec.
  • high current density e.g., >1500 mA/cm 2
  • the high current density can be maintained if the velocity is such that the average time a portion of the substrate surface is covered by the membrane is less than about 0.05 seconds.
  • a very high current density e.g., >2000 mA/cm 2 , for an approximately 1 cm circular membrane, can be maintained if the interfacial velocity v exceeds about 30 to about 40 cm/sec.
  • the high current density e.g., >2000 mA/cm 2
  • the velocity is such that the average time a portion of the substrate surface is covered by the membrane is less than about 0.025 seconds.
  • hydrodynamic forces play a major role in determining the flux of water to the membrane interface and that these forces increase with interfacial velocity regardless of the "dwell-time.” Consequently these dwell times do not, by themselves, provide completely reliable indications of the limiting current density.
  • the quantity of water maintained at the interface must reflect a balance between hydrodynamics of the moving contact and electro-osmotic removal of water.
  • the applied voltage is sufficient to be in the area of the polarization curve where I is highly dependent on V, that is, sufficient to provide a current (or voltage) within the range of current (or voltage) wherein the current density increases at a rate of at least about 50 mA/cm 2 per V, more preferably within the region where the current increases at a rate of at least about 100 mA/cm 2 per V, and even more preferably within the region where the current increases at a rate of at least about 150 mA/cm 2 per V.
  • this may require a voltage of 9 V or greater, preferably 12 V or greater, for example from 13 V to 25V.
  • the process parameters e.g., I, P c , and v
  • I, P c , and v are controlled to avoid the non-continuous process parameter regimes.
  • Increasing velocity and decreasing P c appear to increase the rate of water supply.
  • the average current density be maintained at a fraction of the value where sticking is encountered. For example, for small windows such as were described in our examples, keeping the current density below 0.9 times the current density where audible squeaking is heard may help ensure that the polishing will be smooth, hi one embodiment, the maximum current density during polishing is controlled to between about 0.7 to about 0.85 times the current density where audible squeaking is heard.
  • the operating parameters include I is greater than or equal to about 1 A/cm 2 , n is greater than or equal to about 4 electrons/Cu atom, and A c /A p (contact area of membrane to surface versus area to be polished) is greater than or equal to about 0.02, for example greater than or equal to 0.03, preferably greater than or equal to about 0.05, more preferably greater than or equal to about 0.07.
  • the operating parameters include I is greater than or equal to about 1500 mA/cm 2 , n is between about 4.5 and about 8, and A c /A p (contact area of membrane to surface versus area to be polished) is between about 0.03 and about 0.1.
  • the operating parameters include I is greater than or equal to about 2000 mA cm 2 , n is between about 5 and about 7, and A c /A p (contact area of membrane to surface versus area to be polished) is between about 0.03 and about 0.1.
  • the contact pressure P c is preferably less than 15 psig, preferably less than 10 psig, for example between 0.5 and about 10 psig.
  • the interfacial velocity is preferably greater than 20 cm/sec, preferably advantageously sufficient such that the average time a portion of the substrate surface is covered by the membrane is less than about 0.2 seconds, for example about 0.1 seconds or less, for example about 0.05 seconds or less.
  • P C F Z /A C , where F z is the normal force
  • P c is less than or equal to about 10 psi, preferably is less than or equal to about 5 psi, more preferably is less than or equal to about 2 psi.
  • the hydrostatic pressure (P h ) in the cathode half cell, is related to P c .
  • the pressure inside the half-cell is maintained by a process wherein the contact pressure P c does not increase with contact area A c .
  • This tool pressure is the total force exerted by the membrane on the subsfrate surface, divided by the total surface area to be polished.
  • the contact area is maximized to the extent that the membrane can be moved across a wetted substrate with a layer of low-conductivity fluid disposed between the membrane and the subsfrate surface, for example A c /A p is at least 0.03, e.g., between about 0.04 and 0.2. It is generally beneficial to maximize the ratio of the area of contact of the membrane Ac to the total polishing area A p . However, the beneficial results may not be achieved simply by manufacturing and using very large membranes having large surface areas.
  • the "contact time" for a portion of the substrate to be contacted by a portion of the membrane can be estimated to be the contact area width (-1 cm) divided by the velocity (-20 cm/sec), which gives an average contact time of about 0.05 seconds.
  • the polishing apparatus consist of a plurality of windows, with space between each window such that each window appears to function independently of the other windows, this manner, the ratio A c /A p can be increased without utilizing overly wide membranes.
  • the width of the membrane defined as the length measured in the direction the membrane is advancing, is less than about 4 cm, for example less than about 2.5 cm, for example between about 0.5 cm and about 1.7 cm. This could be done by manufacturing small membrane windows.
  • the use of a window- frame-like structure disposed between a membrane and a subsfrate surface can beneficially divide a larger membrane area into several smaller areas defined by the area in which the membrane projects through the "windows" formed by the window-frame-like structure.
  • the window-frame-like structure can additionally be used to provide sources of water or other low-conductivity fluid to the region between the windows by incorporating or forming regions where the low-conductivity fluid can flow between the windows, and/or to provide surface contact electrical connections to help remove current from the surface, using soft inert metal connectors and/or conductive polymers as a part of the window- frame-like structure.
  • a window-frame-like structure was described in our prior filed applications, which are incorporated herein by reference. Additionally, other methods of introducing low-conductivity fluid to the area within the contact area of the membrane and the substrate surface, which have been described in our previous applications, are incorporated herein by reference. Such methods can include for example a supply of low-conductivity fluid that extends through the membrane and introduces low-conductivity fluid to a small cavity that is, for example, substantially bounded by areas of contact of the membrane to the substrate surface.
  • Such methods also include incorporating regions within the membrane area of contact where the flow of current is substantially blocked, for example where there is an insulator disposed between an area or areas of the membrane and the cathode, hi our prior applications, we suggested a wide variety of fluids, especially polar organic fluids, which may be useful when admixed with the low-conductivity fluids. This is one of several methods we described which may affect the osmotic flow of water across the membrane. This material from our earlier filed applications is incorporated herein by reference. Various combinations of the above can be used, and it is anticipated that having the benefit of this disclosure one of ordinary skill in the art will be able to develop a variety of mechanisms that will introduce predetermined volumes of low- conductivity fluid to the region between the membrane and the substrate surface.
  • the shape of the membrane is such that the contact time between a point on the subsfrate and a portion of the membrane is approximately equal to the contact time for any other point on the substrate that is contacted by other portions of the membrane.
  • the product of the contact time and the current density for a region of the membrane are made to be approximately equal for various parts of the membrane or membranes.
  • the membrane window forms a circle or oval.
  • a shape like an octagon is substantially circular, and a "figure 8" shape, such as shown in Figure 10-B, is substantially oval.
  • This shape is especially useful if both the polishing head holding the membrane and the wafer substrate are both independently rotating. Then, the difference in polishing rate, which will average out as a simple function of the distance from the center to the edge of the circular or oval contact area, can be readily estimated, and the polishing process can be controlled to make sure the entire surface has approximately the same contact time times current density. While the highest RR achieved in these experiments, >800 nm/min, are certainly attractive for planarizing Cu interconnects, practical considerations such as heat dissipation may preclude operating at such high I for at least a portion of the polishing process.
  • the amount of water in the electrolyte may need to be monitored and controlled, though moderate dilution of the electrolyte is not generally viewed as a serious problem. Additionally, as one copper ion and two protons migrate through the membrane and into the electrolyte, positive ions must be reduced at the 4-9 cathode to maintain the charge balance.
  • the concentration of Cu in the elecfrolyte is intentionally maintained at a high value to supress the reduction of protons and evolution of H 2 and assure that Cu +2 reduction (plating) is the only cathodic reaction. Consequently, more Cu +2 is removed at the cathode than is supplied by the anode, so that CuSO 4 in the electrolyte is gradually converted to H 2 SO 4 .
  • the copper ion concentration in the elecfrolyte should be intermittently checked and replenished as needed. Additionally, a more important, though not pathological issue is loss of Cu +2 from the elecfrolyte when the rate of plating at the cathode is greater than the rate of anodic generation of Cu +2 , e.g., when n is greater than 2, due to anodic of water oxidation. This may require adjusting the Cu +2 concentration in the electrolyte reservoir, most likely by means of an independent cell, for example a Cu anode/Air cathode cell. Other methods, including having a membrane with a water permeability sufficient to allow flow of water from the half-cell, are also envisioned.
  • Such a membrane can, but need not, allow the water passing from the half-cell to be admixed with low-conductivity fluid. Less stringent control of ion migration is needed if the water leaving the half cell is not admixed with low-conductivity fluid.
  • Reverse-osmosis to remove excess water from the elecfrolyte
  • the outlet pressure of an electrolyte-circulating pump may be used, by the simple expedient of locating the reverse osmosis membrane near the outlet of the pump. It is clearly beneficial to confrol most of the variables in MMEP.
  • Figure 18 shows an instrumented MMEP tool that allows us to measure (and control) the normal force and drag force at the membrane/work piece interface during the course of the MMEP process.
  • a preliminary measurement was made of the forces on a wet contact as a function of interfacial velocity in the absence of electrochemistry.
  • This response suggests a hydrodynamic lubrication effect where interfacial shear carries a layer of water between the surfaces, and the thickness and corresponding normal stress increase with the strain rate.
  • Nafion ® PFSA membranes (Nl 12, Nl 15, Nl 17, and NEl 135, E. I. DuPont de Nemours and Company, Inc., Wilmington, DE) are non-reinforced films based on Nafion ® PFSA polymer, a perfluorosulfonic acid /PTFE copolymer in the acid (H+) form.
  • Copper coupons (0.016" thick ,110 alloy, 99.9%) were obtained from McMaster- Carr Supply Co., P.O. Box 440, New Brunswick, NJ 08903-0440.
  • Cu damscene wafers 854 AZ and 954 AZ were obtained from International SEMATECH, 2706 Montopolis Drive, Austin, TX 7841. These comprised silicon wafers with 0.5 to 1.0 mm layer of oxide dielectric, covered by 200 nm barrier layer (Ta/N and Ta) and 1.0
  • Example 1 Membrane-mediated Electropolishing of Cu coupons
  • MMEP membrane-mediated electropolishing
  • the half-cell comprised a polypropylene base (1) bolted to a stainless steel faceplate (2) which framed an opening approximately 1.0 cm x 2.5 cm. This opening was covered by a piece of Nafion® membrane (4) (NEl 135, 3.5 mil thick) sealed against the faceplate by means of a silicone rubber gasket (3) approximately 0.5 cm thick.
  • a piece of copper foil (6) was sealed to the base and electiically connected to a wire on the outside of the half-cell.
  • the cavity (5) defined by base, gasket and membrane contained two ports (7) connected to Teflon® tubing (OD1/16") through which the elecfrolyte (0.2 M copper sulfate in 40% phosphoric acid) was continuously re-circulated by means of a peristaltic pump (not shown) from an external reservoir. Electrolyte was pumped through the half-cell at approximately 10 ml/min producing a positive pressure inside the half-cell that caused the membrane to expand (bulge) slightly, forming an liquid-filled elastic bubble or blister.
  • the copper foil served as the cathode and was connected to the negative terminal of the power supply (Princeton Applied Research Model 173 potentiostat/galvanostat, EG&G Instruments, P.O. Box 2565, Princeton, NJ 08540).
  • the work piece (8) consisted of a flat piece of copper sheet metal (l/5"x3”) whose surface had been uniformly milled with 600 grit carborundum abrasive. The work piece was immersed in a shallow bath of deionized water and a voltage of 5.0V was applied between the anode and cathode.
  • Table 1 Changes in roughness R (0.4 mm scans) on milled Cu coupons, before and after polishing using MMEP.
  • Sample Q R ⁇ R ⁇ ⁇ R/ ⁇ co u l/cm" nm nm nm Milled 0.0 310 0 0 1A 1.4 183 127 442 0.29 1 B 2.2 82 228 695 0.33 Milled 0.0 314 0 0 1C 3.0 53 262 948 0.28 1 D 3.1 41 274 980 0.28 Milled 0.0 300 0 0 1 E 3.5 34 266 1106 0.24 1 F 3.7 8 292 1169 0.25
  • Example 2 Comparison of Membrane-Mediated and Conventional Electropolishing Two copper coupons (1.5"x2.0") were milled as in Example 1 and then masked with tape so that only an area of 0.71 cm 2 was exposed. One of these coupons was polished by MMEP using the same procedure as in Example 1. The second coupon was polished by the conventional electropolishing (EP) method.
  • EP electropolishing
  • a silicone rubber gasket 3/8" thick was used to separate the work piece from a sheet of copper foil which served as the cathode.
  • a cylindrical cavity in this gasket provided a volume between the work piece and cathode filled by electrolyte solution that was continuously pumped through cavity.
  • the area of work piece exposed to electrolyte in the EP cell was the same as that polished by the MMEP process.
  • Figure 4 shows the decrease in surface roughness R obtained by polishing milled Cu coupons using EP (prior art) and MMEP.
  • the same voltage, elecfrolyte composition and pumping rate were used for both processes, and the resulting current densities were comparable (0.1 to 0.2 mA/cm 2 ).
  • Roughness R represents an average of the topographic amplitude a for many features with a wide range of lateral dimensions ⁇ .
  • Figure 5 shows the results of polishing by EP (prior art) and by MMEP a work piece with corrugated surface topography consisting of 100 micron ridges separated by 100 micron trenches.
  • Example 3 Comparison of Membrane-Mediated and Conventional Electropolishing for Planarizing Surfaces with Large- ⁇ Features
  • a copper coupon was polished to a mirror finish using jeweler's rouge, then laminated to a dry-film photoresist (Riston ® 9415, E.I. Dupont de Nemours and Company, Inc., Wilmington, DE).
  • the photoresist film was covered with a lithographic negative patterned with 100 micron lines interspersed with 100 micron spaces, and then exposed and developed under conditions recommended by the manufacturer (DuPont Riston ® Printed Circuit Materials, RD 1, New James St., Towanda, PA USA 18848- 9784).
  • the exposed areas on the coupon were etched for 1 minute in a 5% solution of sodium monopersulfate (Sigma- Aldrich, Milwaukee, WI), followed by conventional electropolishing at 2.0 V and 1.5 coul/cm 2 (as described in Example 2).
  • the resist mask was then removed by soaking in dichloromethane, leaving a corrugated surface with alternating 100 micron frenches and 100 micron plateaus.
  • the coupon was masked with tape leaving two circular areas (each 2.3 cm 2 ) exposed.
  • the amplitude of five corrugations in each of three different locations within each area was measured and averaged.
  • One area was then polished via MMEP as in Example 1 at 7.0 V, and the other area was polished via conventional EP as in Example 2 at 3.0 V.
  • the amplitude of corrugations at the same location were re-measured and averaged.
  • Figure 5 These data confirm that MMEP is capable of planarizing large- ⁇ features for which conventional EP is totally ineffective.
  • the dotted line represents Wagner's prediction for ideal (non-convected) EP when ⁇ c > ⁇
  • the dashed line shows Wagner's prediction for EP when ⁇ c ⁇ ⁇ .
  • the close agreement between this function and the MMEP data demonstrate that MMEP is governed by a much smaller boundary layer than predicted by Wagner's model, and indicates that MMEP follows a qualitatively different mechanism than conventional EP.
  • Example 4 Selective Removal of Copper from a Damascene Wafer A 3" x 3" section was cut from a Cu damascene wafer (20 cm in diameter, Sematech 854 pattern 1-63D) which was obtained from EKC Technology (Dupont Electronic Technologies, Hayward, CA). This wafer had been coated with a 0.5 micron layer of dielectric material into which a circuit pattern consisting of lines and contact pads had been lithographically etched. The surface had then been uniformly coated with a thin ( ⁇ 0.1 micron) layer of tantalum followed by a thicker layer (-1 micron) of electroplated copper. Under the optical microscope the underlying circuit pattern remained visible due to variations in surface topography.
  • Example 5 Use of Membrane-mediated Electropolishing for a Variety of Metals and Alloys
  • This example demonstrates the utility of MMEP for polishing various different metals. Polishing was carried out using the same half-cell and methods described in Example 1 but the elecfrolyte composition and voltages were varied as indicated in Table 2. h all cases, the work piece consisted of a flat specimen of the metal whose surface was milled prior to polishing by abrasion with 600 grit sand paper producing an initial mean roughness of approximately 0.3 micron. The surface of the work piece was visually compared before and after MMEP polishing and the detailed surface textures were compared by differential interference contrast (DIC) microscopy (Nikon Optiphot, Optical Apparatus Co.
  • DIC differential interference contrast
  • MMEP of both nickel and 316 stainless steel using water as the low-conductivity solvent produced a mirror-like finish, but the very high values of n (26 and 29 electrons/metal atom) indicated that most of the current must be carried by an anodic process other than metal oxidation. Both of these metals exhibit some catalytic activity for oxidation of water, and in fact gas bubbles were observed to form in the water bath near the MMEP cell consistent with the anodic evolution of O 2 .
  • the much lower concentration of residual Cu ions represents only 2.8% of the total amount of Cu anodically dissolved. Since some of the residual Cu may also have originated from electrolyte permeation, it follows that the efficiency of the MMEP in removing anodized Cu +2 from the work piece was at least 97.2%.
  • Example 7 Spin Polishing Tool for MMEP Planarization of a Cu Damascene Wafer
  • a 3" x 3" section was cut from a Cu damascene wafer (20 cm in diameter, Sematech 854 pattern 1-63D) which obtained from EKC Technology (Dupont Electronic Technologies, 2520 Barrington Court, Hayward, CA 94545-1163).
  • the section was mounted facing up on a rotating mechanical stage (Headway Research, Inc., 3713 Forest Lane, Garland, TX 75042-6928) and the copper blanket layer was connected to the positive terminal of the potentiostat by means of a strip of copper foil taped at one edge.
  • a MMEP cathode half-cell similar to that of Example 1 was fitted with a Nafion ® Nl 17 membrane and filled with elecfrolyte comprising 0.55 M CuSO 4 in 2.2 M H 2 SO under positive pressure via a re-circulating pump.
  • the half-cell was mounted so that the external convex surface of the membrane was held in contact with the surface of the wafer and the area of contact was flooded by a continuous flow of deionized water (approximately 20 ml/min).
  • the wafer was rotated at 100 revolutions per minute so that the area of contact with the half-cell membrane defined a circular "track" approximately 1 cm in width and 5 cm in diameter.
  • the mean thickness of the sample is represented by ⁇ and is the fraction of surface area on the plateaus.
  • ⁇ c represents the convective boundary layer which governs the rate and efficiency of conventional EP in elecfrolyte solutions.
  • the fluxes for removal of metal respectively from plateaus and trenches are inversely proportional to the distance of those surfaces from an effective boundary layer which lies parallel to the sample at a distance ⁇ e from the high points.
  • conventional EP becomes completely ineffective in this limit because the convective boundary layer conforms to the surface topography.
  • Example 8 describes how the MMEP processing conditions may be varied to minimize ⁇ e and thereby maximize the efficiciency of MMEP.
  • Example 8 Effects of Pulsed Voltage on MMEP Planarization Efficiency Polished Cu coupons 3"x3" were lithographically patterned and etched as in Example 3 to provide corrugated surface topography consisting of 100 ⁇ m plateaus separated by 100 ⁇ m trenches with initial amplitudes a 0 ranging from 1 to 3 ⁇ m.
  • each coupon was subjected to MMEP with variations in the operating parameters as summarized in Table 3.
  • the applied voltage was 8V or pulsed between 0V and 8V for repeating time intervals indicated repectively by t(off) and t(on).
  • Interfacial velocity was controlled by setting the angular velocity of the work piece and the radius of the track over which the membrane contacted the work piece. At regular intervals polishing was interrupted following passage of a given amount of charge Q, the mass of Cu removed m was measured and used to calculate the stoichiometric number n (e/Cu) and the change in thickness ⁇ . Amplitudes of corrugations were determined by profilometry at all four quadrants of the polishing area and averagered to generate a plot of a versus ⁇ . The value of ⁇ e in equation 2 was varied to obtain the best fit to the experimental data (see for example Fig. 5).
  • Table 3 Effects of MMEP operating parameters on stoichiometric number and planarization efficiency.
  • the first entry represent a steady-state process in which the voltage was held constant at 8V.
  • the pulsed voltage conditions are seen to provide much greater planarization efficiency (smaller ⁇ e ) and larger values of n than steady-state conditions.
  • These effects can be explained by a mechanism in which the boundary layer consists of a partially dehydrated region of the membrane. Water is inevitably removed from the interface because hydrated Cu +2 ions carry several molecules with them as they migrate across the membrane. Moreover, it has been shown by M. Legras, et al. (Desalination. 147, 351 (2002)), that the diffusion coefficient of water in Nafion ® decreases more than 100 fold when the water content drops below a critical value.
  • Example 9 Planarization of Cu Damascene Wafers with Pulsed Voltage Program
  • Cu damascene wafers with standard test patterns (Sematech 854AZ and 954AZ) were cut into 3"x3" sections. These wafers feature patterns of contact pads and circuit lines of various dimensions etched into a dielectric layer (500 to 1000 nm thick), coated with a thin barrier layer followed by -800 nm of electroplated Cu.
  • the samples were polished by MMEP using the apparatus described in Examples 7 and 8, and illustrated schematically in Figure 6.
  • a pulse program was employed to maintain ⁇ e ⁇ 100 nm. Polishing was interrupted at various intervals to measure the loss of Cu and changes of topographic amplitude a for several circuit features distributed over all four quadrants of the sample.
  • FIG. 8 Two sets of results are shown in Figure 8.
  • the upper plot shows the planarization of circuit pads approximately 100 ⁇ m in width on a 954AZ wafer.
  • the initial amplitude a 0 1000 nm was comparable to the dielectric thickness.
  • Example 10 A Low-Profile MMEP Half-Cell with Polymeric Elecfrolyte
  • a low-profile cathode half-cell was constructed as illustrated in Figure 9.
  • Cupric acetate was dissolved in a 30% solution of poly(ammonium methacrylate) to obtain an homogenerous, deep blue elecfrolyte 1.0 M in Cu +2 , which is highly viscous with the consistence of a grease.
  • Approximately 0.2 ml of this elecfrolyte was spread over about 2 cm 2 of the fiberglass, so that it filled the openings in both fiberglass and underlying Cu screens.
  • a piece of Nl 15 Nafion ® membrane 2.5 x 2.5 cm was then wrapped around the entire assembly as illusfrated schematically in Figure 9. A section of the Cu mesh extending beyond the end of the tube was connected to the negative terminal of the power supply as the cathode.
  • a Cu coupon was attached to the positive terminal of the power supply at an applied voltage of +8V and was immersed in a shallow bath of deionized water.
  • a steady current of about 200 mA was obtained and after passage of 10 coulombs, the surface of the coupon was visibly polished.
  • the high viscosity of the electrolyte prevented flow or leakage of the elecfrolyte outside of the cell.
  • the polymeric nature of the poly(methacrylate) anion prevented any permeation of electrolyte through the membrane.
  • Many parameters are given relative to the contact area A c , and therefore it is advantageous to have a standard method to measure the contact area of a membrane on a surface.
  • a cathode half-cell was constructed, typically with a Nafion® Nl 17 membrane sealed over an oblong window 25 by 18 mm.
  • Figure 10-A is representative of this single- window half-cell configuration. Another configuration used was a 7/8" circle.
  • Electrolyte solution (0.8 M CuSO 4 in 0.5 M H SO 4 ) was pumped through the half-cell by means of a variable speed peristaltic pump (Masterflex model 7021-24) and the hydrostatic pressure P h was measured by means of a pressure transducer connected before the inlet port of the half-cell.
  • a pressure transducer connected before the inlet port of the half-cell.
  • Experiments were carried out with Cu coupons whose surface had been mechanically polished with 1000-grit abrasive.
  • the half-cell was mounted over a flat Cu work piece covered with de-ionized water by means of an adjustable stage incorporating a transducer calibrated to measure the normal force F z between the half-cell and the work piece as a function of the vertical displacement z, which was measured with a micrometer.
  • FIG. 11-A A simplistic drawing of the half-cell moving across a wetted substrate surface is shown in Figure 11-A.
  • a c was determined by measuring the area of the well-defined ellipse on the surface of the work piece, hi Figure 11-B, the area of that surface polished at zero velocity for values of z ranging from 0 to -1.00 mm is clearly evident from the outlines left on the substrate. While there is little polishing within the contact area, a easily visible trench is formed around the periphery of the contact area. Only a fraction of the oblong 25 x 18 mm membrane contacts the subsfrate surface. The contact area at static conditions is presumed to not change significantly when the membrane is moving over the substrate surface.
  • Figure 12-B shows measurements of the contact dimensions as a function of F z for a oblong 25 x 18 mm Nafion ® Nl 17 membrane single- window cathode half-cell having 6.2 psi hydrostatic pressure being exerted by the electrolyte on the membrane.
  • the long and short axes of the elliptical contact increases approximately as (F z ) 1 /9 so that A c increases linearly.
  • the contact area A c is proportional to the product of long and short axes, and is therefore proportional to F z .
  • Another parameter used here is the total area to be polished. In the experiments described subsequently, the polishing path was radial.
  • Figure 11-B also shows the clear outline of a typical radial polishing path, which is shown schematically in Figure 12-A.
  • the polishing area A p was determined by the long axis of the contact and the circumference of the circular "track" covered in a single cycle of rotation. Anodic oxidation was carried out at either a fixed applied voltage or applied current. The respective dependent variable was measured once steady-state conditions were established (approximately 30 sec), and the total integrated charge passed was measured by means of digital coulometer.
  • the removal rate RR was calculated by measuring the mass loss ⁇ m for any given processing time t over a given area of polishing A p using equation (5). Dwell time is the velocity times the length of the short axis of the contact area.(about one half of the long axis) or was the diameter of the area of contact if a circular window was used.
  • Example 11 Formation/Removal Of Copper Oxide/Hydroxide/Carbonate Accumulation
  • the voltages between about 4 V and 8 V were used, there would often be small particulates of material disposed on the polished surface. These were presumed to be copper oxides, though the particles could have also included copper hydroxides and copper carbonates. Such particles do not provide a satisfactory finished surface. Experience suggested these particles were minimized when lower voltages were used.
  • a copper subsfrate was membrane-mediated electro-polished using a single window half cell similar to those described above.
  • the velocity v was 21 cm/sec, P c was 2.2 psi, the area of contact A c was 0.96 cm 2 , and the dwell time was about 0.04 seconds. Dwell time is the average length of the contact area measured in the direction of v, divided by v.
  • the voltage was increased a sequentially from zero, using one volt increments. While we could not be certain when copper oxides/hydroxides/carbonates started to form, by the time the experiment had reached 7 volts the particulate residue was readily visible in the polished section. We continued to increase the voltage, and we surprisingly found that by 11 volts the copper oxide particles were no longer present.
  • the current density at 7 V was about 80 mA/cm 2 , and the current density at 11 V was 150 mA cm 2 .
  • the polarization curve is shown in Figure 23-C.
  • the dwell time varied from about 0.005 seconds at the lowest load (-150 grams) to about 0.029 seconds at the highest load (-750 grams), where velocity was constant and the change in dwell time reflects the change in the length of the short axis.
  • Example 13 Effect Of Hydrostatic Pressure On Normal Force F z
  • a cathode half-cell was constructed with a Nafion ® Nl 17 membrane sealed over an oblong window 25 by 18 mm.
  • Electrolyte solution (0.8 M CuSO 4 in 0.5 M H2SO 4 ) was pumped through the half-cell by means of a variable speed peristaltic pump (Masterflex model 7021-24) and the hydrostatic pressure P was measured by means of a pressure fransducer connected before the inlet port of the half-cell.
  • FIG. 20- A shows results of the measurements of normal force versus distance (a constant plus z) for three hydrostatic pressures P h of the elecfrolyte.
  • Figure 20-B shows the calculated variation in contact area A c with normal load and hydrostatic pressure P of the electrolyte.
  • the normal load F z should equal the contact pressure P c times the contact area A c .
  • the values of Ph in Fig. 20 included a pressure drop between the transducer and the half-cell.
  • the data in Figure 20-B show the contact pressure P c was substantially independent of the area of contact A c .
  • F z changed proportionally to A ⁇ .
  • the hydrostatic pressure P h in the half-cell was generated by the resistance to flow of electrolyte while pumping electrolyte into the half-cell at a constant rate.
  • the resistance to flow was primarily due to the inlet and outlet, as the area within the cell was so large that we expect there to be very little pressure drop within the cell, and consequently very little change in the pressure when the dimensions of the half-cell are changed a small amount. So changing the distance between the base of the half-cell and the substrate surface would not increase the resistance of flow of elecfrolyte. Such a assembly will result in the half cell advantageously self-correcting to the same pressure after movement toward the subsfrate surface (to increase the contact area between the membrane and the substrate surface).
  • the pressure within the half-cell in such a system can be changed by varying the flow rate of electrolyte circulation through the half cell, by altering the backpressure at the at the cell outlet while maintaining constant flow rate, and/or by changing the size of a restricted opening at the point where electrolyte exits the cell.
  • both P and P c will become extremely sensitive to variations in z. Both will increase as the half-cell is moved closer to the substrate surface (to increase the contact area between the membrane and the substrate surface).
  • Example 15 Effect Of Velocity And Contact Area On Nominal Current Density
  • the 9 9 contact area was varied between about 0.05 cm and about 0.6 cm by changing z.
  • the dwell time varied from about 0.008 seconds to about 0.036 seconds.
  • Example 16 Effect Of Interfacial Velocity v On Polarization Curve
  • a flat Cu work piece was mounted on a rotating turntable in an MMEP apparatus with a cathode half-cell containing, for the data shown in Figure 21, a Nafion ® Nl 17 membrane sealed over a round window 7/8" in diameter.
  • the half-cell was held at a fixed vertical displacement relative to the work piece so as to maintain a constant contact area Ac.
  • the work piece was rotated about an axis offset from the area of contact so as to establish various interfacial velocities.
  • interfacial velocity we mean the average velocity of the membrane relative to, and substantially parallel to, the subsfrate surface, wherein the average velocity is the velocity seen by the membrane surface at the center of the membrane contact area with the substrate surface (within the contact area, points nearer or further from the center of rotation experience velocities slightly lower or higher respectively than the average velocity).
  • v was 21, 39 and 56 cm/sec (dwell times of about 26 milliseconds, 14 milliseconds, and 9.6 milliseconds, respectively).
  • V > 25V and v > 39 cm sec (dwell time 14 milliseconds), I is seen to increase steeply with V and water oxidation became increasingly important as indicated by n>4. It can be seen that, for voltages ⁇ 8V, the values of I decrease systematically with increasing v.
  • ohmic resistance can also be introduced when the rate of water consumption between the membrane and the subsfrate surface exceeds the rate of water supply, which we believe leads to a reduction in n, a reduction of I, and eventually squeaking and sticking of the membrane to the substrate, with accompanying damage to both the membrane and the substrate.
  • the portion that sticks may be, for example, that portion that is downstream of the direction of travel of the membrane, winch therefore has proportionately longer time to dehydrate as the current flow results in water and proton transfer through the membrane. Squeaking is expected to result in a rough surface and in excessive membrane wear, and is therefore not desired.
  • Example 17 Effect Of Interfacial Velocity v On Polarization Curve
  • the slope of the polarization curve obtained at v 37 cm/sec, from 7 V to 21 V, was 120 mA cm 2 per volt.
  • the slope of the polarization curve obtained at v 56 cm/sec, from 7 V to 21 V, was over 150 mA/cm 2 per volt.
  • the slope dl/dV of the polarization curve does not necessarily have to be monotonically increasing to provide high RR, though it often is in preferred embodiments of the invention, but if the slope is not increasing then advantageously the slope of dL/dV is greater than 100 mA/cm 2 and the voltage V is greater than about 10V, preferably greater than about 14V.
  • Example 18 Effect Of Interfacial Velocity v On Polarization Curve
  • Figure 14-B shows the variation of nominal current density with voltage for a single-window half-cell using a 2 mil experimental expanded PTFE web impregnated with perfluorosulfonic acid polymer material similar to that used in Nafion ® type membranes.
  • the PTFE membrane would reduce friction, and would also decrease wear, relative to Nafion ® membranes.
  • the nembrane was only 2 mils thick, compared to the 7 mil Nafion ® membranes we normally used.
  • the z was controlled to give a modestly low contact area of about 0.3 cm 2 .
  • the polarization curves were determined at a variety of interfacial velocities from 12 to 64 cm/sec, i.e., dwell times from about 0.046 to about 0.008 seconds.
  • the 12 cm/sec experiment was the lowest constant velocity tested. While the slope dl/dV of the polarization curve monotonically increased, the 2 mil membrane developed sufficient adhesion at voltage V of only 2.4V to stop the rotor.
  • the polarization curve was again characterized by a low slope that increased monotonically with increasing voltage, though again the increase after about 8V was very small.
  • the initial slope at the intermediate velocity, v 30 cm/sec and dwell time is about 0.018 seconds, was greater than the initial slope at the higher velocities. While it appears that unlike other Examples described herein the slope at higher velocities did not eventually increase to provide a current density exceeding that for intermediate velocities, this may be partially the result of terminating testing at 13 volts, where other tests extended the voltage to over 20 volts.
  • Example 19 Effect Of Contact Pressure P £ On Polarization Curve
  • the sheared water layer must develop a hydrostatic pressure comparable to the contact pressure P c .
  • the thickness of the water interlayer should decrease as P c is increased.
  • the quantity of water maintained at the interface must reflect a balance between hydrodynamics of the moving contact and electro-osmotic removal of water.
  • We investigated whether the stiffness of the membrane blister might play a role in the hydrodynamics by measuring the effects of elecfrolyte hydrostatic pressure on the polarization curve. At applied voltages below about 10 V, the higher pressure inside the half cell provides a higher current density compared to lower pressures.
  • FIG. 22 shows polarization curves similar to those the previous example, but in wliich P c was varied from 2.5 psig to 14 psig while v was maintained constant at 39 to 40 cm/sec. The shape of the polarization curves suggests that higher P c , for example the approximately 14 psig, has a similar effect to lower interfacial velocities of about 20 cm/sec.
  • Example 19 illustrates the effects of contact pressure on the MMEP polarization curve.
  • lower pressure we mean below 20 psig, for example below 15 psig, preferably below 10 psig, and more preferably below about 5 psig. While we polished copper substrates with a contact P c of 2 psig, we expect that a contact pressure of 0.5 psig or 1 psig will be operative. Generally, the higher the contact pressure Pc, the higher should be the interfacial velocity V. However, a half-cell operating at a low P c , for example at 2.5 psig, can polish at high velocities or at low velocities.
  • Example 20 Dependence of Removal Rate And Stoichiometric Number On Voltage
  • Example 20 contains a set of experiments were performed to determine the dependence of the copper removal rate RR and of the stoichiometric number n on voltage.
  • Figure 16-A shows the dependence of removal rate (RR) and stoichiometric number (n) on current density I, where the current density varied from about 600 mA/cm 2 to about 2800 mA/cm 2 .
  • Figure 16-B shows the dependence of I and n on voltage for the single- window half-cell experiment of Figure 16-A.
  • Figure 16-C shows the variation of removal rate and current density versus voltage for the single- window half-cell experiment of Figure 16-A.
  • I is very sensitive to V, therefore experiments in this regime were carried out under applied current.
  • the voltage increased from 14.5 and 18V, while RR increased by about 250%.
  • RR varies with the ratio I/n (see Equation 6).
  • the measurements of n confirm an increase of only about 50% beyond random error. Both I and n increased with increasing V in the interval between 12 and 20V, but the proportional increase in I was much greater than that in n.
  • n will be greater than 3.9 if either the current density is greater than about 1000 mA cm 2 , or if the voltage is greater than about 15 volts. Further, the data from Figures 1 -A and 16-B suggest that, for this system, n will be greater than 5 if either the current density is greater than about 1500 mA/cm 2 , or if the voltage is greater than about 16 volts.
  • Example 21 Comparative Performance Of Single And Dual- Window Half-Cells - Dependence Of Area Of Contact On Removal Rate And On Stoichiometric Number n
  • Figure 10-B shows a dual window half cell.
  • a dual- window cathode half-cell with two 17 x 25 mm Nafion ® membranes was used to polish a Cu coupon secured on the turntable of the laboratory spin-polishing apparatus.
  • the tests described here compared the performance of a single window half-cell polisher of Figure 10- A with a dual window half-cell polisher of Figure 10-B.
  • This example compared the key process performance parameters n and RR of a dual window polishing cell and compared these to the same parameters measured in Example 20 above.
  • the process parameters for the single window and the dual window experiments are shown below for ease of comparison.
  • Dividing by A p this corresponds to a tool pressure of 0.43 psi, or dividing by A c it corresponds to a contact pressure of 7.3 psi.
  • the actual contact pressure is ⁇ 20 psi.
  • Example 22 The Relationship Between Normal Force F? Exerted On The Substrate Versus The Resultant Drag Force Fj
  • P also referred to as the "down force”
  • the down force is defined as the normal force exerted by the pad onto the wafer (substrate) divided by the area of the wafer.
  • the damage done to substrates is believed to be the result of drag forces operating in a direction substantially parallel to the direction of the interfacial velocity.
  • transducers to measure both the normal force and the drag force exerted by the membrane as it polished a copper substrate.
  • Figures 18-A, 18-B, and 18-C show an instrumented MMEP tool adapted for use on a 200 mm base and showing a cathode half-cell mounted on dual cantilever fixture, wherein displacements of the first cantilever are proportional to the z-axis (normal) force on the work piece, while displacements of the second cantilever are proportional to the interfacial drag force.
  • the drag forces are initially about equal to the no ⁇ nal forces at very low velocities, e.g., from about 0 to about 13 cm/sec.
  • the results show a dramatic reduction in the drag force with increasing v.
  • the drag forces quickly dropped off, and were less than one half the normal force at a velocity of about 50 cm sec.
  • This response suggests a hydrodynamic lubrication effect where interfacial shear carries a layer of water between the surfaces, and the thickness and corresponding normal stress increase with the strain rate.
  • having a high current density will increase the drag force, as shown in the following Example.
  • the data suggests that when the polishing is started or stopped, lower drag forces will be generated if the interfacial velocity is greater than 20 cm sec, preferably greater than 40 cm/sec, before the membrane contacts the subsfrate. Alternately, startup can occur with the membrane disposed over a margin of the substrate that does not have a pattern of circuits disposed thereon.
  • Example 23 Demonstration That Copper Ions Electropolished From A Surface Pass Through Membrane And Into Electrolyte.
  • De-ionized water was delivered via two ports located on either side of the membrane window at a flow rate of 840 ml/min. The flow of de-ionized water was recovered prior to contact with the membrane and found to have a conductivity of 0.3 ⁇ S/cm.
  • the same stream of de-ionized water was then allowed to flow across the surface of a flat copper work piece, 200 mm in diameter, mounted on the polishing tool and rotating at 190 rpm.
  • the effluent stream recovered from the surface of this work piece was found to have a conductivity of 0.5 ⁇ S/cm.
  • the effluent stream recovered from the surface of this work piece was found to have a conductivity of 0.9 ⁇ S/cm.
  • the effluent stream recovered from the surface of this work piece was found to have a conductivity of 0.8 ⁇ S/cm.
  • the conductivity of the de-ionized water effluent was not significantly increased by contact with the half-cell or by current flow during polishing of the work piece.
  • This example confirms that the membrane is an effective barrier against diffusion of electrolyte out of the half-cell and prevents contamination of the de-ionized water and the work piece.
  • the conductivity of the effluent is low enough that it becomes more practical to directly recycle this water through the de-ionizer rather than utilizing a fresh water supply. This procedure reduces the cost of generating de-ionized water and also eliminates a waste stream.

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